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#### Question 1:

(a) On what basis did mandeleev arrange the elements in his periodic table?
(b) On what basis are they arranged now?

(a) Mendeleev arranged the elements in his periodic table on the basis of atomic masses.
(b) In the modern periodic table, the elements are arranged on the basis of atomic numbers.

#### Question 2:

State whether the following statements are true of false:

(a) Newlands divided the elements into horizontal rows of eight elements each.
(b) According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic numbers.
(c) The elements in a group have consecutive atomic numbers.

(a) False
Newlands divided the elements into horizontal rows of seven elements each.

(b) False
According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses.

(c) False
The elements in a period have consecutive atomic numbers.

#### Question 3:

Name the Russian chemist who said that the properties of elements are a periodic function of their atomic masses.

Dmitri Mendeleev was a Russian chemist who said that the properties of elements are a periodic function of their atomic masses.

#### Question 4:

Rewrite the following statements after correction, if necessary:

(a) Groups have elements with consecutive atomic numbers.
(b) Periods are the horizontal rows of elements.
(c) Isotopes are the elements of the same group.

(a) Periods have elements with consecutive atomic numbers.
(b) The statement is correct.
(c) The statement is correct.

#### Question 5:

Name the scientists who gave the following laws in the early classification of elements:

(a) Law of octaves

(a) John Newlands is the scientist who gave the Law of Octaves in the early classification of elements.
(b) Johann Dobereiner, a German chemist gave the Law of Triads in the early classification of elements.

#### Question 6:

A, B and C are the elements of a Dobereiner's triad. If the atomic mass of A is 7 and the of C is 39, what should be the atomic mass of B?

A, B and C are the elements of a Dobereiner's triad. If the atomic masses of A and C are 7 and 39 respectively, then according to Dobereiner's triad law, the atomic mass of B is the average of the atomic masses of A and C.

So, the average of the atomic masses of A and C =$\frac{7+39}{2}=\frac{46}{2}=23$
Therefore, the atomic mass of B is 23.

#### Question 7:

X and Y are the two elements having similar properties which obey Newland's laws of octaves. How many elements are there in-between X and Y?

There are six elements between X and Y.
Since X and Y are the two elements having similar properties and obey the Newland's law of octaves, the number of elements between X and Y, including both, must total eight.

#### Question 8:

What was the Mendeleev's basis for the classification of elements?

The atomic mass was Mendeleev's basis for the classification of elements.

#### Question 9:

In the classification of the then known elements, Mendeleev was guided by two factors. What are those two factors?

In the classification of the then known elements, Mendeleev was guided by two factors, which are:
1. Increasing atomic masses
2.Grouping together of elements having the same properties

#### Question 10:

Name two elements whose properties were predicted on the basis fo their positions in Mendeleev's periodic table.

Gallium (Ga) and scandium (Sc) are the two elements whose properties were predicted on the basis of their position in Mendeleev's periodic table.

#### Question 11:

The three elements predicted by Mendeleev from the gaps in his periodic table were known as eka-boron, eka-aluminimum and eka-silicon. What names were given to these elements when they were discovered later on?

(i) Eka-boron was named scandium (Sc).

(ii) Eka-aluminium was named gallium (Ga).

(iii) Eka- silicon was named germanium (Ge).

#### Question 12:

Name two elements whose properties were predicted on the basis of their positions in Mendeleev's periodic table.

Germanium (Ge) and Scandium (Sc) are the two elements whose properties were predicted on the basis of their position in Mendeleev's periodic table.

#### Question 13:

State one example of a Dobereiner's triad, showing in it that the atomic mass of middle element is half-way between those of the other two.

Lithium (Li), sodium (Na) and potassium (K) with atomic masses 7, 23, 39  constitute a Dobereiner's triad. The atomic mass of sodium (middle element) is half-way between those of the other two. This can be seen in the illustration below:

Average of the atomic masses of lithium and potassium =

#### Question 14:

Which group of elements could be placed in Mendeleev's periodic table later on, without disturbing the order? Give reason.

Noble gases are the group of elements which could be placed in Mendeleev's periodic table later on, without disturbing the order. This is  because they are inert (unreactive) elements, as their valence shells are completely filled with electrons. Therefore, Mendeleev decided to include these elements in a separate group.

#### Question 15:

Fill in the following blanks with suitable words:

(a) The basis for modern periodic table is ...............
(b) The horizontal rows  in a periodic table are called ...............
(c) Group 1 elements are called ...............
(d) Group 17 elements are known as ...............
(e) Group 18 elements are called ...............
(f) According to Newland's classification of elements, the properties of sulphur are similar to those of oxygen because sulphur is the ............... element starting from oxygen.

(a) The basis for the modern periodic table is the atomic number.

(b) The horizontal rows  in a periodic table are called periods.

(c) Group 1 elements are called alkali metals.

(d) Group 17 elements are known as halogens.

(e) Group 18 elements are called noble gases.

(f) According to Newland's classification of elements, the properties of sulphur are similar to those of oxygen because sulphur is the eighth element, starting from oxygen.

#### Question 16:

(a) What is meant by (i) a group, and (ii) a period, in a periodic table?
(b) How many periods and groups are there in the long form of periodic table?
(c) Give two examples each of (i) group 1 elements (ii) group 17 elements (iii) group 18 elements.

(a) (i) A group is a vertical column of elements in a periodic table.
(ii) A period is a horizontal row of elements in a periodic table.

(b) There are seven periods and eighteen groups in the long form of the periodic table.

(c) (i) Lithium (Li) and sodium (Na) are two elements of group 1.
(ii) Chlorine (Cl) and bromine (Br) are two elements of group 17.
(iii) Neon (Ne) and argon (Ar) are two elements of group 18.

#### Question 17:

(a) In the modern periodic table, which are the metals among the first ten elements?
(b) What is the significance of atomic number in the modern classification of elements? Explain with the help of an example.

(a) Lithium and beryllium are the metals among the first ten elements in the modern periodic table.

(b) The significance of atomic numbers in the modern periodic table is that it helps in arranging the elements according to their electronic configuration. Since, the number of valence electrons is the same, elements placed in the same group show similar chemical properties.
For example, if we take the element lithium (Li), its atomic number is 3. Its electronic configuration is (2,1). As it has one valence electron, it is placed in the first group of the periodic table. Similarly, all the elements having one valence electron will be placed in group 1, and they show similar chemical properties. In this manner, classification of elements is made according to the electronic configuration of the elements.

#### Question 18:

(a) How were the positions of isotopes of an element decided in the modern periodic table?
(b) How were the positions of cobalt and nickel resolved in the modern periodic table?
(c) where should hydrogen be placed in the modern periodic table? Give reason for your answer.

(a) The isotopes of  elements are not given a separate place in the periodic table. This is because isotopes have the same atomic number and a different atomic mass, and the modern periodic table is based on the atomic number of elements. Hence, to give it a separate place, the elements should be arranged according to their atomic mass.

(b) As per Mendeleev's classification, elements are arranged in an increasing order of atomic masses, but cobalt, with a higher atomic mass was placed before nickel. Cobalt has an atomic mass of 58.9 and nickel has an atomic mass of 58.7, which is slightly lower than that of cobalt.  Mendeleev could not explain this point. This problem was resolved in the modern periodic table. According to the modern periodic law, elements are arranged in the increasing order of their atomic numbers. The atomic numbers of cobalt and nickel are 27 and 28 respectively. So, cobalt with a lower atomic number should come before nickel.

(c) Generally, hydrogen is treated as a special element and placed alone at the head of the periodic table. The position of hydrogen in the periodic table is unclear because it resembles both the alkali metals and halogens in some of its properties. However, it is placed in the first group, above all the alkali metals in the modern periodic table because its electronic configuration is similar to that of alkali metals. But due to the small size of hydrogen, it exhibits properties different from that of the alkali metals.

#### Question 19:

(a) On which side of the periodic table will you find metals?
(b) On which side of the periodic table will you find non-metals?
(c) What is the name of those elements which divide metals and non-metals in the periodic table?

(a) On the left side of the periodic table, we find metals.

(b) On the right side of the periodic table, we find non-metals.

(c) Metalloids are the names of those elements which divide metals and non-metals in the periodic table.

#### Question 27:

(a) Why do we classify elements?
(b) What were the two criteria used by Mendeleev to classify the elements in his periodic table?
(c) Why did Mendeleev leave some gaps in his periodic table?
(d) In Mendeleev's periodic table, why was there no mention of noble gases like helium, neon and argon?
(e) Would you place the two isotopes of chlorine, CI-35 and CI-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.

(a) Elements are classified into certain groups in such a manner that elements belonging to the same group exhibit similar properties. This eases the study of elements, as we can reduce the study to a few groups of elements rather than studying the properties of all the 115 elements known at present, which is very difficult. This is the reason for classification of elements.

(b) The two criteria used by Mendeleev to classify the elements in his periodic table are:
(i) Increasing atomic masses
(ii) Grouping of elements which exhibited similar properties, placed under the same vertical column (group).

(c) Mendeleev left some gaps in his periodic table for the elements which were not known at that time, to ascertain that elements having the same properties fell in the same vertical column or group, without disturbing the arrangement of previous elements.

(d) There was no mention of noble gases like helium, neon and argon in Mendeleev's periodic table as those gases were not known at that time.

(e) The isotopes of chlorine Cl-35 and Cl-37 should be placed in the same slot because their chemical properties are the same. The two isotopes of chlorine have the same atomic number and chemical properties. We know that the arrangement of elements is made on the basis of atomic number in the periodic table. And chemical properties of the element depend on the atomic number of the atom.

#### Question 28:

(a) State Mendeleev's periodic law.
(b) What chemical properties of elements were used by Mendeleev in creating his periodic table?
(c) State any three limitations of Mendeleev's classification of elements.
(d) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table?
(e) Which group of elements was missing from Mendeleev's original periodic table?

(a) Mendeleev's periodic law: According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses. This means, when the elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at a regular interval of the periods.

(b) Mendeleev  used the formula of the oxides and hydrides produced by the elements as the basic chemical properties of elements, in creating his periodic table.

(c) Limitations of Mendeleev's classification of elements are:

i) Mendeleev's periodic law could not explain the position of isotopes in the periodic table.
ii) Mendeleev's periodic law could not assign a proper position to the element hydrogen in the periodic table.
iii) Mendeleev's periodic law failed to explain the wrong order of atomic masses of some elements.

(d) Germanium (Ge) and scandium (Sc) are the two other elements besides gallium, which have since been discovered, for which Mendeleev had left gaps in his periodic table.

(e) Group 18 consisting of noble gases was missing from Mendeleev's original periodic table as the noble gases were not known at that time.

#### Question 29:

(a) State modern periodic law.
(b) How does the electronic configuration of the atom of an element relate to its position in the modern periodic table?
(c) How could the modern periodic law remove various anomalies of Mendeleev's periodic table? Explain with examples.
(d) Is it possible to have an element having atomic number 1.5 placed between hydrogen and helium?
(e) Name the scientist who prepared modern periodic table.

(a) Modern periodic law: According to the modern periodic law, the properties of elements are a periodic function of their atomic numbers. This means, if the elements are arranged in an increasing order of their atomic numbers, the elements exhibiting similar properties will repeat after regular intervals of periods.

(b) The elements are arranged in an increasing order of atomic number, and it helps in arranging the elements according to their electronic configuration. Since elements having the same number of valence electrons show similar chemical properties, they are placed under the same group.

(c) As the modern periodic law is based on the atomic numbers of elements, all the anomalies of Mendeleev's periodic table are removed. For example:

1. The position of isotopes could be explained, which the Mendeleev's periodic table failed to explain. As all the isotopes of an element have the same atomic number, they can be placed under the same group in a periodic table.
For example:  Cl-35 and Cl-36, which are the isotopes of Chlorine with the same atomic number 17, they can be placed in the same group of the periodic table.

2. Mendeleev could not explain why cobalt was placed before nickel. This problem was solved by the modern periodic law. The atomic numbers of cobalt and nickel are 27 and 28 respectively. Hence, cobalt with a lower atomic number was placed before nickel in the modern periodic table.

(d) It is not possible to place an element with an atomic number of 1.5 between hydrogen and helium. This is because elements are placed in the increasing order of their atomic numbers. Atomic numbers are always whole numbers and cannot be in decimals, as half an electron or half a proton cannot exist .

(e) Niels Bohr is the scientist who prepared the modern periodic table.

#### Question 30:

In Mendeleev's periodic table, gap was not left for one of the following elements. This element is:

(a) gallium
(b) beryllium
(c) germanium
(d) scandium

(b) Beryllium.

In Mendeleev's periodic table, a gap was left for gallium, scandium and germanium.

#### Question 31:

The Newland's law of octaves for the classification of elements was found to be applicable only up to the element:

(a) potassium
(b) calcium
(c) cobalt
(d) phosphorus

(b) Calcium.
The Newlands' law of octaves for the classification of elements was applicable only up to the element calcium.

#### Question 32:

According to Mendeleev's periodic law, the elements were arranged in the periodic table in the order of:

(a) decreasing atomic numbers
(b) increasing atomic numbers
(c) decreasing atomic masses
(d) increasing atomic masses

(d) Increasing atomic masses.
According to Mendeleev's periodic law, the elements in the periodic table were arranged in the order of increasing atomic masses.

#### Question 33:

The three elements having chemical symbols of Si, B and Ge are:

(a) all metals
(b) all non-metals
(c) all metalloids
(d) Si is metalloid, B is metal and Ge is non-metal

(c) All metalloids
The three elements having the chemical symbols Si, B and Ge are metalloids.

#### Question 20:

(a) Name three elements that have a single electron in their outermost shells.
(b) Name two elements that have two electrons in their outermost shells.
(c) Name three elements with completely filled outermost shells.

(a) Lithium (Li), sodium (Na) and potassium (K) are three elements (metals of group 1)  that have only a single electron in their outermost shell.

(b) Magnesium (Mg) and calcium (Ca) are two elements (metals of group 2)  that have two electrons in their outermost shell.

(c) Helium (He), neon (Ne) and argon (Ar) are three elements (inert gases of group 18) that have a completely filled outermost shell.

#### Question 21:

What is Debereiner's law of triads? Explain with the help of one example of a Dobereiner's triad.

According to Dobereiner's law of triads, when elements are arranged in increasing order of their atomic masses, a group of three elements with similar chemical properties is obtained. This group is called a triad. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two elements.

For example, the set of elements: calcium (Ca), strontium (Sr) and barium (Ba), with atomic masses 40, 88 and 137 respectively form a triad because they have similar chemical properties, and the atomic mass of  strontium is approximately equal to the average of the atomic masses of calcium and barium.
Arithmetic mean of atomic masses of calcium and barium = $\frac{40+137}{2}=88.5$

#### Question 22:

What is Newlands' law of octaves? Explain with an example.

According to Newlands' law of octaves, when elements are arranged in increasing order of their atomic masses, the properties of the eighth element are a repetition of the properties of the first element. Newlands divided the elements into horizontal rows. Each horizontal row had 7 elements.

For example, let us take a row from Newlands' classification of elements. If we take the elements Li, Be, B, C, N,O, F and Na, lithium (Li) is the first element, and sodium the eighth. It has been found that the properties of sodium are a repetition of the properties of lithium. In short, both lithium and sodium have similar chemical properties.

#### Question 23:

(a) Did Dobereiner's triads also exist in the columns of Newlands' law of octaves? Explain your answer.
(b) What were the limitations of Dobereiner's classification of elements?
(c) What were the limitations of Newlands' law of octaves?

(a) Yes, Dobereiner's triads also existed in the columns of Newlands' law of octaves. This can be explained by taking the second column of the Newlands' classification of  elements as an example. The second column has the elements lithium (Li), sodium (Na) and potassium (K), which form a Dobereiner's triad.

(b) Limitations of Dobereiner's classification of elements:
It could identify only three triads from the elements which were known at that time.  It failed to arrange all the elements which were known then in the form of triads of elements with the same chemical properties.

(c) Limitations of Newlands' law of octaves:
1. Newlands' law of octaves was applicable only up to the element calcium and not beyond that, i.e, it only works for lighter elements.

2. According to Newlands, only 56 elements existed in nature and there would be no elements discovered in the future. However, this assumption was proved wrong with the discovery of several elements later on which did not fit into Newlands' law of octaves.

3. A unique slot was not given to all the elements, as Newlands  put two elements which have different properties together in a single slot.

4. The element iron which resembles cobalt and nickel in its properties was given a position far away from them.

#### Question 24:

(a) State the periodic law on which Mendeleev's periodic table table was based. Why and how was this periodic law changed?
(b) Explain why, the noble gases are placed in a separate group.

(a) According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses. This means that when elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at regular periods.
However, Henry Moseley, a scientist, showed that the atomic number of an element is a more fundamental property and a better basis to classify elements than the atomic mass. The atomic number increases consistently by 1, from one element to the next, whereas the atomic mass does not change regularly. Also, the atomic number of an element does not change and remains fixed. This led to a change in Mendeleev's periodic law.

(b) Noble gases are placed in a separate group of the periodic table because they are inert elements. They are unreactive as  their valence shells are completely filled with electrons. Their properties are different when compared to all the other elements.

#### Question 25:

(a) State the merits of Mendeleev's classification of elements.
(b) Describe two anomalies of Mendeleev's periodic classification of elements.

(a) The merits of Mendeleev's classification of elements:

1. Mendeleev's periodic law predicted the existence of certain elements such as gallium, scandium and germanium, which were not discovered at that time.

2. Mendeleev's periodic table could also predict the properties of various elements based on their position in the periodic table.

3. Mendeleev's periodic table could place group 18 elements, called Noble gases in the periodic table, when they were discovered.

(b) Anomalies of Mendeleev's periodic classification of elements:

1. Mendeleev's periodic classification of elements could not explain the position of isotopes in the periodic table: Since, isotopes of the same metal have similar chemical properties but different in their atomic masses. So, the isotopes of the same element should be given different place in periodic table as Mendeleev's periodic table was arranged in increasing order of their atomic masses. However, isotopes were not given separate place in his periodic table.

2.Mendeleev's periodic classification of elements could not assign a proper place for the element,hydrogen. Since, hydrogen resembled both alkali metals and halogens in some of the properties.

#### Question 26:

(a) How do the properties of eka-aluminium element predicted by Mendeleev compare with the actual properties of gallium element? Expalin your answer.
(b) What names were given by Mendeleev to the then undiscovered elements (i) scandium (ii) gallium, and (iii) germanium?

(a) The properties of eka-alumiunium, predicted by Mendeleev, which was undiscovered then, were almost similar to the actual properties of the element discovered later on. The comparison table below shows the similarities in the properties of the undiscovered element then and the actual element (Gallium).

 Property Eka-Aluminium Gallium Atomic Mass 68 69.7 Density 5.9g/cm3 5.94g/cm3 Melting point low 30.2oC (low) Formula of oxide Ea2O3 Ga2O3 Formula of chloride EaCl3 GaCl3

(b) (i) Scandium (Sc) was known as eka-boron.
(ii) Gallium(Ga) as eka-aluminium.
(iii) Germanium (Ge) as eka-silicon.

#### Question 34:

In Mendeleev's periodic table, gaps were left for the elements to be discovered later on. An element which found a vacant place in the periodic table later on is:

(a) Be
(b) Si
(c) Ge
(d) Se

(c) Ge.
Germanium (eka- silicon) is the element which found a vacant place in the periodic table later on.

#### Question 35:

The three imaginary elements X, Y and Z represent a Dobernener's triad. If the atomic mass of element X is 14 and that of element Y is 46, then the atomic mass of element Z will be:

(a) 28
(b) 60
(c) 78
(d) 72

(c) 78
The atomic mass of the element Z is 78.
According to  Dobereiner's law of triads, atomic mass of $\mathrm{Y}=\frac{\mathrm{X}+\mathrm{Z}}{2}$
Substituting the atomic masses we get:

$92=14+\mathrm{Z}$

Therefore, $\mathrm{Z}=78$.

#### Question 36:

The atomic numbers of four elements P, Q, R and S are 6, 8, 14 and 16 respectively. Out of these, the element known as metalloid is:

(a) P
(b) Q
(c) R
(d) S

(c) R.
R (Si) is a metalloid since its atomic number is 14 and it has 4 valence electrons.

#### Question 37:

Which of the following statement is correct in regard to the classification of elements?

(a) Elements in modern periodic table are arranged on the basis of increasing atomic masses.
(b) Elements in Mendeleev's periodic table are arranged on the basis of increasing atomic numbers.
(c) In modern periodic table, the element nickel of lower atomic mass is kept before the element cobalt of higher atomic mass.
(d) In modern periodic table, the isotopes of chlorine having different atomic masses are kept in the same group.

(d) In the modern periodic table, the isotopes of chlorine, having different atomic masses are kept in the same group.
The isotopes of chlorine, having different atomic masses but same atomic numbers are kept in the same group in the modern periodic table.

#### Question 38:

Which of the following statement about the modern periodic table is correct?

(a) It has 18 horizontal rows known as periods.
(b) It has 7 vertical columns known as periods.
(c) It has 18 vertical columns known as groups.
(d) It has 7 horizontal rows known as groups.

(c) It has 18 vertical columns known as groups.
In the modern periodic table, there are 18 vertical columns known as groups.

#### Question 39:

An element X forms an oxide X2O3. In which group of Mendeleev's periodic table is this element placed?

(a) group II
(b) group III
(c) group V
(d) group VIII

(b) Group III.
The element which forms the oxide X2O3 is placed in group 3 of the periodic table.

#### Question 40:

The modern periodic table was prepared by:

(a) Dobereiner
(b) Newlands
(c) Bohr
(d) Mendeleev

(c) Bohr.
The modern periodic table of elements was prepared by Neils Bohr.

#### Question 41:

The atomic particle whose number in the atoms of an element always remains the same and which forms the real basis for the modern classification of elements is:

(a) electron
(b) proton
(c) neutron
(d) meson

(b) Proton.
Proton is an atomic particle whose number in the atoms of an element remains the same always, and forms the real basis for the modern classification of elements.

#### Question 42:

The atomic masses of three elements X, Y and Z having similar chemical properties are 7, 23 and 39 respectively.

(a) Calculate the average atomic mass of elements X and Z.
(b) How does the average atomic mass of elements X and Z compare with the atomic mass of element Y?
(c) Which law of classification of elements is illustrated by this example?
(d) What could the elements X, Y and Z be?
(e) Give another example of a set of elements which can be classified according to this law.

(a) The atomic masses of three elements X, Y and Z are 7, 23 and 39 respectively. The average atomic mass of X and Z is nothing but the arithmetic mean of the atomic masses of X and Z.

Therefore, the average atomic mass of  X and Z = $\frac{7+39}{2}=23$.

(b) From the above calculation, we observe that the average atomic mass of the elements X and Z is equal to the atomic mass of the element Y.

(c) Dobereiner's law of triads is illustrated by the above example.
According to Dobereiner's law, when elements are arranged in an increasing order of atomic masses, a three element group (triads), having the same chemical properties is formed. The atomic mass of the middle element is equal to the average of the atomic masses of the other two elements.

(d) The elements X,Y and Z , having atomic masses 7, 23, 39 are lithium (Li), sodium (Na) and potassium (K). They belong to the alkali metal group (group 1), with the same valency of 1 and exhibit similar chemical properties.

(e) The elements calcium (Ca), strontium (Sr) and barium (Ba) are a set of alkali earth metals (group 2 elements), with atomic masses 40,88 and 137 respectively, which are classified according to Dobereiner's law of triads.

#### Question 43:

In the following set of element, one element does not belong to the set. Select this element and explain why it does not belong:
Calcium, Magnesium, Sodium, Beryllium

In the given set of elements, i.e., calcium, magnesium, sodium and beryllium, sodium does not belong to the set.  This is because sodium is a group 1 element and the remaining three elements​ belong to group 2. Elements of the same group show similar chemical properties.

#### Question 44:

In the following set of elements, one element does not belong to the set. Select this element and state why it does not belong:
Oxygen, Nitrogen, Carbon, Chlorine, Fluorine

In the given set of elements, i.e., oxygen, nitrogen, carbon, chlorine and fluorine, chlorine does not belong to the set. The element chlorine belongs to the third period while the remaining elements belong to the second period.

#### Question 45:

Can the following groups of elements be classified as Dobereiner's triads?

(a) Na, Si, Cl
(b) Be, Mg, Ca
(Atomic masses : Be 9 ; Na 23 ; Mg 24 ; Si 28 ; Cl 35.5 ; Ca 40)

(a) The atomic masses of Na, Si and Cl are 23, 28 and 35.5 respectively.
According to Dobereiner's law of triads, when elements are arranged in the increasing order of their atomic masses, a three element group with similar chemical properties is obtained. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two.

The above set of elements does not form a triad because arithmetic mean of  the atomic masses of Na and Cl = $\frac{23+35.5}{2}=29.25$
This value is approximately equal to the atomic mass of silicon, but they do not form a triad because the elements do not exhibit similar chemical properties.

(b) The set of elements Be, Mg and Ca with atomic masses 9, 24 and 40 respectively, form a triad  because they have similar chemical properties and the atomic mass of magnesium (Mg) is  approximately equal to the average of the atomic masses of beryllium (Be) and calcium (Ca).
The arithmetic mean of the atomic masses of  Be and Ca =$\frac{9+40}{2}=24.5$ which is approximately equal to 24.

#### Question 46:

Consider the following elements:
Na, Ca, Al, K, Mg, Li

(a) Which of these elements belong to the same period of the periodic table?
(b) Which of these elements belong to the same group of the periodic table?

(a) Sodium (Na), magnesium (Mg) and aluminium (Al) belong to the same(third) period of the periodic table.

(b) Lithium (Li), sodium (Na) and potassium (K) belong to the same group(group 1) of the periodic table.

#### Question 47:

Which element has:

(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as its first shell?

(a) Neon with atomic number 10.
Electronic configuration:- Shells:                   K L
Electrons arranged:   2 8

(b) Magnesium is the element with the electronic configuration of 2,8,2 as its atomic number is 12, since the atomic number is the sum of all the electrons present in the electronic configuration.
Electronic configuration:- Shells:                   K L M
Electrons arranged:   2 8 2

(c) Silicon is an element with a total of three shells and four electrons in its valence shell. Its atomic number is 14 and the electronic configuration is 2,8,4.
Electronic configuration:- Shells:                   K L M
Electrons arranged:   2 8 4

(d) Boron is an element with a total of two shells, with three electrons in its valence shell. Its atomic number is 5 and electronic configuration is 2,3.
Electronic configuration:- Shells:                   K L
Electrons arranged:   2 3

(e) Carbon is the element with twice as many electrons in its second shell as its first shell. The atomic number of carbon is 6 and its electronic configuration is 2,4.
Electronic configuration:- Shells:                   K L
Electrons arranged:   2 4

#### Question 48:

Consider the following elements:
Ca, Cl, Na, I, Li, Ba, Sr, K, Br

Separate these elements into three groups (families) of similar properties. State one property in each case on the basis of which you have made your choice.

(i) Lithium (Li), sodium (Na) and potassium (K) are three metals of group 1 having a valency of 1.
(ii) Calcium (Ca), strontium (Sr) and barium (Ba) are three metals of group 2 having a valency of 2.
(iii) Chlorine (Cl), bromine(Br) and iodine (I) are three non-metals of group 17 which are known as halogens, and have seven valence electrons.

#### Question 49:

Mendeleev predicted the existence of certain elements not known at that time and named two of them as eka-aluminium, and eka-silicon.

(a) Name the element which has taken the place of (i) eka-aluminium, and (ii) eka-silicon
(b) Mention the period/periods of these elements in the modern periodic table.
(c) Write the group/ groups of these elements in the modern periodic table.
(d) Classify these elements as metals, non-metals or metalloids.
(e) How many valence electrons are present in the atoms of each of these elements?

(a) (i) Gallium is the element which has taken the place of eka-aluminium.

(ii) Germanium is the element which has taken the place of eka-silicon.

(b) Gallium and Germanium are placed in the 4th period of the periodic table.

(c) Gallium belongs to group 13 and Germanium belongs to group 14 in the periodic table.

(d) Gallium is a metal and Germanium, a metalloid.

(e) Gallium, which belongs to group 13, has 3 valence electrons in its atom, and germanium which belongs to group 14 has 4.

#### Question 50:

A part of the early classification of elements has been given below:

 H Li Be B C N O F Na Mg Al Si P S

(a) Which law of classification of element is illustrated by the above arrangement of elements?
(b) Name the scientist who proposed such a classification of elements.
(c) Why is such a classification of elements compared with a characteristics of musical scale?
(d) State one limitation of this classification of elements.

(a) Newlands' law of octaves is illustrated by the above arrangement of elements.

(b) John Newlands is the scientist who proposed this classification of elements.

(c) This classification is compared with a characteristic of the musical scale because the repetition of the properties of elements is just like the repetition of the eighth note in an octave of music.

(d) Limitation: Newlands' law of octaves could only be applied up to the element calcium.

#### Question 1:

Give alongside is a part of the periodic table :
As we move horizontally from left to right:

 Li Be B C N O F Na Mg Al Si P S Cl

(i) What happens to the metallic character of the elements?
(ii) What happens to the atomic size?

(i) The metallic property of elements decreases as we move horizontally from left to right.

(ii) The atomic size of elements decreases as we move horizontally from left to right.

#### Question 2:

How would the tendency to gain electrons change on moving from left to right in a period of the periodic table?

The tendency to gain electrons increases on moving from left to right in a period of the periodic table, because the metallic character decreases from left to right, and the non-metallic character increases.

#### Question 3:

How would the tendency to lose electrons change as we go from left to right across a period of the periodic table?

The tendency to lose electrons decreases as we go from left to right across a period of the periodic table.
This happens because the atomic size decreases from left to right and the force of the nuclear attraction on the outermost electron increases. So, it becomes difficult for the atom to lose electrons.

#### Question 4:

(a) How does the chemical reactivity of alkali metals vary on going down in group 1 of the periodic table?
(b) How does the chemical reactivity of the halogens vary on going down in group 17 of the periodic table?

(a) The chemical reactivity of alkali metals increases on going down in group 1 of the periodic table.
(b) The chemical reactivity of halogens (non-metals) decreases on going down in group 17 of the periodic table.

#### Question 5:

What property do all elements in the same column of the periodic table as boron have in common?

All elements in the same column of the periodic table as boron have the same valency of 3.

#### Question 6:

What property do all the elements in the same group of the periodic table as fluorine have in common?

All elements in the same group of the periodic table as fluorine are halogens. Halogens are placed in group 17 which have seven valence electrons. Hence, their valency is 1.

#### Question 7:

(a) What is the number of valence electrons in the atoms of first elements in a period?
(b) What is the usual number of valence electrons in the atoms of the last element in a period?

(a) The number of valence electrons in the atoms of first elements in a period is 1.

(b) The usual number of valence electrons in the atoms of the last element (inert gas) in a period is 8. Helium (He) is the only inert gas having 2 valence electrons.

#### Question 8:

State whether the following statement is true or false:
On going down in a group of the periodic table, the number of valence electrons increases.

False
On going down in a group of the periodic table, the number of valence electrons remains the same.

#### Question 9:

What is the major characteristic of the first elements in the periods of the periodic table? What is the general name of such elements?

The first element in every period of the periodic table have 1 valence electron. The elements are known as alkali metals.

#### Question 10:

How do the atomic radii of elements change as we go from left to right in a period of the periodic table?

As we go from left to right in a period of the periodic table, the atomic radii decrease.

#### Question 11:

What happens to the metallic character of the elements as we go down in a group of the periodic table?

The metallic character of elements increases as we go down in a group of the periodic table.

#### Question 12:

How does the number of valence electrons vary on moving from left or right:

(i) in the first period of the periodic table?
(ii) in the second period of the periodic table?

i) In the first period of the periodic table, the number of valence electrons increases from 1 to 2, on moving from left to right.

ii) In the second period of the periodic table, the number of valence electrons increases from 1 to 8 on moving from left to right.

#### Question 13:

How does the valency of elements change of moving from left to right in the third period of the periodic table?

The valency of the elements increases from 1(Na) to 4 (Si) and then decreases to 0 (Ar), on moving from left to right in the third period of the periodic table.

#### Question 14:

How does the valency of elements vary in going down a group of the periodic table?

The valency of elements remains the same on going down a group of the periodic table.

#### Question 15:

Name the element which is in:

(a) first group and third period.
(b) seventeenth group and second period.

(a) Sodium (Na) is the element which is in the first group and the third period.

(b) Fluorine (F) is the element which is in the seventeenth group and the second period.

#### Question 16:

How do electronic configurations of elements change in second period of periodic table with increase in atomic numbers?

 Element in the second period Li (3) Be (4) B (5) C (6) N (7) O (8) F (9) Ne(10) Electronic configuration 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

#### Question 17:

Arrange the following elements in increasing order of their atomic radii:
Li, Be, F, N

The elements in the increasing order of their atomic radii are as follows:

F<N<Be<Li.

#### Question 18:

Arrange the following elements in the increasing order of their metallic character:
Mg, Ca, K, Ga

The elements in the increasing order of their metallic character are as follows:
Ga < Mg < Ca < K

#### Question 19:

Rewrite the following statements after correction, if necessary:

(i) Elements in the same period have equal valency
(ii) The metallic character of elements in a period increases gradually on moving from left to right.

(i) Elements in the same group have equal valency.

(ii) The metallic character of the elements in a period decreases gradually on moving from left to right.

#### Question 20:

Fill in the blanks in the following statements:

(a) The horizontal rows in a periodic table are called ...............
(b) In going across a period (right to left) in periodic table, the atomic size of the atom ...............
(c) On moving form right to left in the second period, the number of valency electrons ...............
(d) On going down in a group in the periodic table, the metallic character of elements ...............
(e) The tendency to gain an electron ............... on moving down in a group of the periodic table.

(a) The horizontal rows in the periodic table are called periods.

(b) On moving across a period (right to left) in the periodic table, the atomic size of an atom increases.

(c) On moving from right to left in the second period, the number of valence electrons decreases.

(d) On moving down in a group of the periodic table, the metallic character of elements increases.

(e) The tendency to gain an electron decreases on moving down in a group of the periodic table.

#### Question 21:

Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

Electronic configuration of nitrogen (atomic number 7) :
Shells:      K L
Electrons: 2 5

Electronic configuration of phosphorus (atomic number 15) :
Shells:      K L M
Electrons: 2  8  5

The element nitrogen will be more electronegative because of the smaller size of its atom compared to phosphorus. Since, nitrogen has a smaller atomic radius than phosphorus, the attraction of its nucleus towards the incoming electron is more than phosphorus. Therefore, nitrogen accepts electrons more easily.

#### Question 22:

An element X belongs to group 2 and another element Y belongs to group 15 of the periodic table:

(a) What is the number of valence electron in X?
(b) What is the valency of X?
(c) What is the number of valence electrons in Y?
(d) What is the valency of Y?

(a) The number of valence electrons in element X is 2 because this element belongs to group 2.

(b) The valency of the group 2 element X is 2.

(c) Valence electrons of group 15:
Group number = Valence electrons + 10
15 = Valence electrons + 10
Valence electrons = 15 - 10 = 5

(d) Valency of the group 15 element Y is 3, that is, (8 - 5).

#### Question 23:

(a) What is a period in a periodic table? How do atomic structures (electron arrangements) change in a period with increase in atomic numbers from left to right?
(b) How do the following change on going from left to right in a period of the periodic table?
(i) Chemical reactivity of elements
(ii) Nature of oxides of elements

(a) A period is a horizontal row of elements in a periodic table. As the atomic number increases from left to right in a periodic table, the size of the atom decreases.

(b) (i) On moving from left to right in a period of the periodic table, the chemical reactivity of the elements first decreases and then increases.
For example, in the third period elements comprising of Na, Mg, Al, Si, P, S and Cl, sodium (Na) is very reactive as it has one valence electron, and can therefore, lose its electron easily. The chemical reactivity gradually decreases as we go to aluminium (Al) and silicon (Si). This is because the number of valence electrons increases, making it difficult to lose electrons. Moving further right in the period towards non-metals, the chemical reactivity again gradually increases. Phosphorus (P) has 5 valence electrons and needs 3 electrons to complete its octet. Sulphur (S) has 6 valence electrons and needs 2 more electrons to complete its octet. Chlorine (Cl) has 7 valence electrons and needs only one more electron to complete its octet. As chlorine can easily accept an electron as compared to phosphorus and sulphur, the chemical reactivity increases from phosphorus to chlorine.

(ii) On moving from left to right in a period of the periodic table, the basic nature of oxides decreases and their acidic nature increases. For example, taking the third period elements again, the oxides of sodium are highly basic whereas those of chlorine are highly acidic.

#### Question 24:

(a) How does the size of atoms (atomic size) generally vary in going from left to right in a period of the periodic table? Why does it vary this way?
(b) What happens to the metallic character of the elements as we move from left to right in a period of the periodic table?

(a) The size of the atoms decreases on moving from left to right in a periodic table. This happens because the number of electrons and protons also increases and the nucleus of the atom becomes more positively charged. The nucleus exerts greater force of attraction on the electrons and pulls them tightly. As the nucleus pulls the outermost electrons towards it, the size of the atom (or the atomic radius) decreases.

(b) The metallic (electropositive) character of the elements decreases as we move from left to right and the non-metallic (electronegative ) character increases in a period of the periodic table.

#### Question 26:

(a) How does the electropositive character of elements change on going down in a group of the periodic table?
(b) State how the valency of elements varies (i) in a group, and (ii) in a period, of the periodic table.

(a) The electropositive character of the elements increases on going down in a group of the periodic table.

(b) (i) In a group, all the elements will have the same valency because all the electrons have the same number of electrons in their outermost shell.

(ii) In a period, different elements have different valencies. On moving from left to right in a period, the valency of the elements increases from 1 to 4 and then decreases to 0.

#### Question 27:

(a) What is the fundamental difference in the electron configurations between the group 1 and group 2 elements?

(b) On the basis of electronic configuration, how will you identify:
(i) chemically similar elements?
(ii) the first element of a period?

(a)

 Group I Electronic configuration Group II Electronic configuration Li (Lithium) 2, 1 Be (Beryllium) 2,2 Na (Sodium) 2, 8, 1 Mg (Magnesium) 2,8,2 K (Potasium) 2, 8, 8, 1 Ca (Calcium) 2,8,8,2

(b) (i) Elements having the same number of valence electrons in their atoms will be chemically similar because the chemical properties of an element depend on the number of electrons present in the outermost shell (valence electrons) of an atom of the element.

(ii) The first element of each period has only one outermost electron. For example, if an element has an electronic configuration of (2,1), we understand that the element belongs to group 1.

#### Question 28:

(a) What is the usual number of valence electrons and valency of group 18 elements of the periodic table?
(b) What happens to the number of valence electrons in the atoms of elements as we go down in a group of the periodic table?

(a) The usual number of valence electrons of group 18 elements is 8. Therefore, the valency of the group 18 elements is: 8-8=0.

(b) On moving down in a group of the periodic table,  the number of valence electrons in the atoms of elements remains the same.

#### Question 29:

(a) What is the main characteristic of the last elements in the periods of the periodic table? What is the general name of such elements?
(b) What is the number of elements is : (a) 1st period, and (b) 3rd period, the modern periodic table?

(a) The last elements in the periods of the periodic table have their valence shells completely filled with electrons. They are known as noble or inert gases. Their valency is zero.

(b) The number of elements in (a) 1st period is 2 and (b) 3rd period is 8 in the modern periodic table.

#### Question 30:

(a) How does the atomic size vary on going down from top to bottom in a group of the periodic table? Why does it vary this way?
(b) Lithium, sodium and potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements? Explain your answer.

(a) The atomic size increases on moving from top to bottom in a group of the periodic table. This is because, on moving down in a group, new shells of electrons are added.

(b) Lithium, sodium and potassium are all metals which belong to group 1, and have 1 valence electron in their atoms. They all have the same electronic configuration. Therefore, each element shows similar chemical properties when reacting with water and liberates hydrogen gas.

#### Question 31:

(a) How does the tendency to lose electrons change as we go down in group 1 of the periodic table? Why does it change this way?
(b) How does the tendency to gain electrons change as we go down in group 17 of the periodic table? Why does it change this way?

(a) In group 1 of the periodic table, the tendency to lose electrons increases on moving from top to bottom. This happens because an electron shell gets added at every stage and, the atomic radius increases. As the size of the atom increases, the distance between the valence electrons and the nucleus increases. This reduces the nuclear attraction force, causing the atom to lose its valence electrons easily, and form cations.

(b) In group 17 of the periodic table, the tendency to gain electrons decreases on moving from top to bottom. On moving down in a group, an electron shell gets added at every stage. As a result, the atomic radius increases. Since the size of the atom goes on increasing, it becomes difficult for the nucleus to attract the incoming electron, due to which the atom cannot form anions easily.

#### Question 32:

(a) Why does the size of the atoms progressively become smaller when we move from sodium (Na) to chlorine (Cl) in the third period of the periodic table?
(b) Helium and neon are unreactive gases. What, if anything, do their atoms have in common?

(a) The size of atoms decreases progressively, on moving from sodium (Na) to chlorine (Cl) in the third period because the atomic number of the elements increases. Likewise, the number of electrons and protons in the atoms also increases. As the positive charges on the nucleus increase, the outermost electrons are held more tightly by the nucleus. Therefore, the size of the atom decreases.

(b) The atoms of helium and neon have their valence shells completely filled with electrons. The number of valence electrons is 8. So, they do not show any reactivity.

#### Question 33:

(a) In the modern Periodic Table, why does cobalt with higher atomic mass of 58.93 appear before nickel having lower atomic mass of 58.71?
(b) Why could no fixed position be given to hydrogen in Mendeleev's periodic table?

(a) In the modern periodic table, elements are arranged according to their increasing atomic number. The atomic numbers of cobalt (Co) and nickel (Ni) are 27 and 28, respectively. Therefore, although cobalt (Co) has higher atomic mass, it appears before nickel (Ni) in the periodic table.

(b) Mendeleev's periodic law could not assign a fixed position to hydrogen in the periodic table because hydrogen resembled both alkali metals (Group 1) and halogens (Group 17) in some of its properties. Hydrogen reacts with  metals to form ionic compounds called hydrides and also with non-metals to form covalent compounds.

#### Question 34:

(a) What are the periods and groups in a periodic table? Give two characteristics of each.
(b) In terms of electronic configurations, explain the variation in the size of the atoms of the elements belonging to the same period and same group.
(c) Given alongside is a part of the periodic table. As we move vertically downward from Li to Fr:

 Li Be Na K Rb Cs Fr Ra
(i) What happens to the size of atoms?
(ii) What happens to their metallic character?
(d) Name two properties of elements whose magnitudes change when going from top to bottom in a group of the periodic table. In what manner do they change?
(e) Rewrite the following statement after correction, if necessary:
Groups have elements with consecutive atomic numbers.

(a) Periods are the horizontal rows of elements and Groups, the vertical columns of elements in the periodic table.

Characteristics of groups:
1. Elements of the same group have the same number of valence electrons.
2. Elements of the same group exhibit similar chemical properties as they have the same valency.

Characteristics of periods:
1. Elements of the same period have the same number of shells.
2. The valency of elements in the same period changes, and therefore, the elements in a period exhibit different chemical properties.

(b) The size of the atom increases on moving from top to bottom in the same group because at every step, new shells of electrons are added.
In a period, the size of the atom decreases on moving from left to right. This is because the atomic number of the elements increases on moving from left to right. Thereby, the number of electrons and protons also increases. Due to the increment in the number of protons, the nuclear charge also increases and the electrons get held more tightly to the nucleus.

(c) When we move vertically down from Li to Fr:
(i) The size of the atom increases.
(ii) The metallic character of the elements increases.

(d) On moving from top to bottom in a group , the size of the atoms and the metallic character of the elements increases.

(e) Periods have elements with consecutive atomic numbers.

#### Question 25:

(a) Explain why:
(i) All the elements of a group have similar chemical properties.
(ii) All the elements of a period have different chemical properties.

(b) The atomic radii of three elements X, Y and Z of a period of the periodic table are 186 pm; 104 pm and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.

(a) (i) All the elements of a group have similar chemical properties because they have the same number of electrons in their outer most shell.
(ii) All the elements of a period have different chemical properties because the number of valence electrons for each varies.

(b) The elements in the increasing order of atomic numbers in the period is X

#### Question 35:

(a) Explain why, the first period of the modern periodic table has only two elements whereas second period has eight elements
(b) Why do elements in the same group show similar properties but the elements in different groups show different properties?
(c) For each of the following triads, name the element with the characteristics specified below:

 Elements Least atomic radius Chemically least reactive (i) F, Cl, Br (ii) Li, Na, K .................... .................... .................... ....................
(d) State one reason for keeping fluorine and chlorine in the same group of the periodic table.
(e) What are the merits of the modern periodic table of elements?

(a) The first period has only two elements because the first shell (K shell) can take a maximum of only two electrons. The second period has eight elements because the second shell (L shell) can accommodate a maximum of 8 electrons. The number of elements in a period depends upon the number of shells.

(b) The properties of an element depends on the number of valence electrons present in an atom of the element. Elements of a particular group have similar electronic configurations and the same number of valence electrons in their atoms. Hence, all the elements belonging to a certain group show similar properties. However, the electronic configurations of elements change on moving from left to right in a periodic table. Different groups have different valencies. Therefore, elements in different groups show different properties.

(c)

 Elements Least atomic radius Chemically least reactive (i) F, Cl, Br F Br (ii)Li, Na, K Li Li

(d) Fluorine and chlorine have a valency of 1 and show similar chemical properties. So, they are placed in the same group.

(e) The merits of the Modern Periodic Table of Elements are as follows:
1. It is based on the atomic number of elements.

2. The modern periodic table clearly explains the reason behind elements showing similar properties in a group and dissimilar properties in different groups.

3. This also explains the reason for the periodicity in the properties of elements.

4. The modern periodic table explains the reason for a repetition in the properties of elements after 2,8,18 and 32 elements.

5. In the modern periodic table, there are no anomalies in the arrangement of elements.

#### Question 36:

(a) What is a group in the periodic table? In which part of a group would you separately expect the elements to have (i) the greatest metallic character (ii) the largest atomic size?
(b) In what respects do the properties of group 1 elements differ from those of group 17 elements? Explain with examples by taking one element from each group.
(c) From the standpoint of atomic structure, what determines which element will be the first and which the last in a period of the periodic table?
(d) Explain why, the properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table.
(e) What are the advantages of the periodic table?

(a) A group is a vertical column of elements in the periodic table.
In a group:
(i) The greatest metallic character is expected by elements in the lowest part of the group, as the metallic (electropositive) character increases on going down in a group.
(ii) The largest atomic size is found at the bottom of the group, as the atomic size increases on moving down in a group of the periodic table.

(b) Let us explain the properties by taking the example of sodium (Na) in group 1 and chlorine (Cl) in group 17:

 Properties Group 1 Group 17 i) Atomic number Na, atomic number is 11 Cl, atomic number is 17 ii) Size of the atom Na has a smaller size than Cl Cl has a larger atomic size comparatively iii) Metallic character Na is a metal, shows more metallic character Cl is non-metallic iv) Valence electron Number of valence electrons is 1 for Na metal Number of valence electrons is 7 for Cl v)Electronegativity/ Electropositivity Na is electropositive in nature Cl is electronegative in nature vi) Chemical reactivity Na is basic in nature Cl is acidic in nature

(c) The number of valence electrons in atoms determines which element will be the first and which the last in a period of the periodic table.

(d) The electronic configurations are repeated because 2, 8, 18 and 32 are the maximum number of electrons that can be placed in the four atomic shells, namely, K, L, M and N. The properties of the elements are repeated after 2, 8, 18 and 32 because the electronic configurations of elements are repeated after 2, 8, 18 and 32.

(e) Advantages of the periodic table:
(i) The study of elements and its properties is made easy.
(ii) By knowing the position of the element in the periodic table, prediction of the type of compounds that the elements form becomes easy.
(iii) The study of chemistry has been made easy in schools and colleges by using the periodic table chart as a teaching-aid.

#### Question 37:

Which of the following statements is not a correct statements about the trends when going from left to right across the periods of the periodic table?

(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.

(c) The atoms lose their electrons more easily.
On moving from left to right in the periodic table, the tendency of an atom to lose electrons decreases.

#### Question 38:

The electronic configuration of the atom of an element X is 2, 8, 4. In modern periodic table, the element X is places in :

(a) 2nd group
(b) 4th group
(c) 14th group
(d) 8th group

(c) 14th group
Group number = 10 + 4
= 14
Hence, the element X is placed in the 14th group.

#### Question 39:

The atomic number of an element is 20. In modern periodic table, this element is placed:

(a) 2nd period
(b) 4th period
(c) 3rd period
(d) 1st period

(b) 4th period.

Electronic configuration : K L M N
2 8  8  2
Hence, from the electronic configuration, it is clear that the outermost electron goes into the 4th shell. So, the element would be placed in the 4th period.

#### Question 40:

Five elements A, B, C, D and E have atomic numbers of 2, 3, 7, 10 and 18 respectively. The elements which belong to the same period of the periodic table are:

(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E

(b) B, C, D:
B, C and D, with atomic numbers 3,7 and 10 respectively, belong to the same period of the periodic table. This is because the elements B, C and D have the same valence shell (L shell).

#### Question 41:

The elements A, B, C, D and E have atomic numbers 9, 11, 17, 12 and 13 respectively. The pair of elements which belongs to the same group of the periodic table is:

(a) A and B
(b) B and D
(c) A and C
(d) D and E

(c) A and C
Electronic configuration of A (9) : K L
2  7
Electronic configuration of C (17): K L M
2  8 7
Group number of A and B = valence electron + 10
=  7+10 = 17.

#### Question 42:

Which of the following element would lose an electron easily?

(a) Mg
(b) Na
(c) K
(d) Ca

(c) K
K would lose an electron easily as it is a group 1 metal whose atomic number is greater than that of Na, which also belongs to group 1. Mg and Ca are group 2 metals and the tendency to lose electrons decreases on moving from left to right in a period of periodic table.

#### Question 43:

Which of the following element does not lose an electron easily?

(a) Na
(b) F
(c) Mg
(d) Al

(b) F
F is the element that does not lose an electron easily, as it is a non-metal (halogen) belonging to group 17 of the periodic table.

#### Question 44:

Where would you locate the element with electronic configuration 2, 8 in the modern periodic table?

(a) group 8
(b) group 2
(c) group 18
(d) group 10

(c) Group 18.

The element with an electronic configuration of (2, 8) belongs to group 18 (noble or inert gases) of the periodic table, as the elements in this group have their valence shell completely filled with electrons.
Group number = valence shell + 10
= 8+10 = 18.

#### Question 45:

An element which is an essential constituent of all organic compounds belongs to following group of modern periodic table:

(a) group 4
(b) group 14
(c) group 15
(d) group 16

(b) Group 14.
Carbon is the essential constituent of all organic compounds and belongs to Group 14 of the modern periodic table.
Electronic configuration of carbon (6): K L
2 4
Group number = valence shell+10
= 4+10 = 14.

#### Question 46:

Which of the following is the valence shell for the elements of second period of the modern periodic table?

(a) M shell
(b) K shell
(c) L shell
(d) N shell

(c) L shell.
L is the valence shell for elements of the second period of the modern periodic table, since the period number is decided by the number of valence shells.

#### Question 47:

The element which has the maximum number of valence electrons is:

(a) Na
(b) P
(c) Si
(d) Al

(b) P.
Phosphorus (P) has the maximum number of valence electrons, equal to 5.
Electronic configuration of P (15): K L M
2  8  5
Sodium (Na) has 1, Silicon has 4 and Aluminium (Al) has 3.

#### Question 48:

The correct increasing order of the atomic radii of the elements oxygen, fluorine and nitrogen is:

(a) O, F, N
(b) N, F, O
(c) O, N, F
(d) F, O, N

(d) F, O, N
The increasing order of the atomic radii of oxygen, fluorine and nitrogen is fluorine, oxygen and nitrogen. This is because the atomic radius decreases from left to right in a period.

#### Question 49:

The atomic numbers of the elements Na, Mg, K and Ca are 11, 12, 19 and 20 respectively. The element having the largest atomic radius is:

(a) Mg
(b) Na
(c) K
(d) Ca

(c) K.
Potassium (K) with an atomic number of 19 has the largest atomic radius. This is because sodium and potassium are elements of group 1. On moving from top to bottom in a group, the atomic radius increases. Magnesium and calcium are elements of group 2. The size of the atomic radius decreases on moving from left to right in a period of the periodic table

#### Question 50:

Which of the following are the correct characteristics of isotopes of an element?
(i) same atomic mass
(ii) same atomic number
(iii) same physical properties
(iv) same chemical properties

(a) (i), (ii) and (iv)
(b) (ii), (iii) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)

(d) (ii) and (iv).
Isotopes of an element have the same atomic number and the same chemical properties.

#### Question 51:

The correct formula of the oxide of Eka-aluminium element predicted by Mendeleev was:

(a) EaO3
(b) Ea3O2
(c) Ea2O3
(d) EaO

(c) Ea2 O3
The correct formula for the oxide of eka-aluminium, as predicted by Mendeleev, is Ea2O3.

#### Question 52:

The element which can from an acidic oxide should be the one whose atomic number is:

(a) 6
(b) 16
(c) 12
(d) 19

(b) 16.
The element which can form an acidic oxide should be the one with atomic number 16. Sulphur has an atomic number of 16 and sulphur oxide is acidic.

Note: Option (a) 6: carbon also forms an acidic oxide but the acidic strength of the oxides of sulphur (16) is greater than that of carbon (6).

#### Question 53:

The element which forms a basis oxide has the atomic number of:

(a) 18
(b) 17
(c) 14
(d) 19

(d) 19.
Potassium (K) forms a basic oxide and has an atomic number of 19. The atomic numbers of 18,17 and 14 correspond to acidic oxides.

#### Question 54:

Which one of the following does not increase while moving down the group of the periodic table?

(b) metallic character
(c) valence electrons
(d) shells in the atoms

(c) Valence electrons
The number of valence electrons does not increase while moving down the group in the periodic table.

#### Question 55:

On moving from left to right in a period of the periodic table, the atomic number of elements increases. What happens to the size of atoms of elements on moving from left to right in a period?

(a) increases
(b) decreases
(c) remain the same
(d) first increases then decreases

(b) Decreases.
The size of the atom decreases on moving from left to right in a period of the periodic table. With an increment in the atomic number, both the proton number and the nuclear positive charge increase. As the nucleus attracts the outermost electron, the size of the atom decreases.

#### Question 56:

Which of the following set of elements is written correctly in the order of their increasing metallic character?

(a) Mg, Al, Si
(b) C, O, N
(c) Na, Li, K
(d) Be, Mg, Ca

(d) Be, Mg, Ca.
Be, Mg and Ca are elements of group 2, written in order of their increasing metallic character. On moving down in the group, the metallic character increases.

#### Question 57:

The atomic numbers of the three elements X, Y and Z are 2, 6 and 10 respectively.
(i) Which two elements belong to the same group?
(ii) Which two elements belong to the same period?

(i) In order to determine the group of the elements, let us first write down the electronic configuration of the elements from their atomic numbers.

Element              Atomic Number                       Electronic Configuration
Shells:       K        L
X                                  2                                          2
Y                                  6                                          2        4
Z                                 10                                         2        8

In this case, X and Z are in the same group. Generally, elements which contain the same number of valence electrons belong to the same group of the periodic table. However, here, the element X (2) fills its outermost shell with two electrons and Z (10), with 8 electrons. Both belong to group 18 (noble gas).

(ii) The elements which have the same valence shell belong to the same period in the periodic table. We can see from the electronic configurations of Y and Z , that they have the same valence shell (L). Therefore, elements Y and Z belong to the same period in the periodic table.

#### Question 58:

An atom has the electron structure of 2, 7.
(a) What is the atomic number of this atom?
(b) To which of the following would it be chemically similar?
7N, 15P, 17Cl, 18Ar
(c) Why would you expect it to be similar?

(a)  The atomic number of the atom is 9 (electronic configuration 2, 7), since it is the sum total of the electrons present in an atom.

(b) The atom would be chemically similar to 17Cl (electronic configuration 2,8,7).

(c) They are similar because both have the same number of valence electrons (7) in their atoms.

#### Question 59:

Consider the following elements:
20Ca, 8O, 18Ar, 16S, 4Be, 2He
Which of the above elements would you expect to be:

(i) very stable?
(ii) in group 2 of the periodic table?
(iii) in group 16 of the periodic table?

(i) 18Ar and 2He are very stable elements, as their valance shells are completely filled. They belong to the group of inert or noble gases.

(ii) 20Ca and 4Be are the two elements that belong to group 2 of the periodic table. This is because their valency is 2 and they are electropositive.

(iii) 8O (2,6) and 16S (2,8,6) are the two elements that belong to group 16 of the periodic table.
Group number = Valence shell + 10
= 6 + 10 = 16

#### Question 60:

In each of the following pairs, choose the atom having the bigger size:

(a) Mg (At. No.12) or Cl (At. No.17)
(b) Na (At. No. 11) or K (At. No. 19)

(a) Mg has a bigger size than Cl because the atomic size decreases on moving from left to right in a period of the periodic table. Mg and Cl belong to group 3 of the periodic table.

(b) K has a bigger size than Na because the atomic size increases on going down in a group of the periodic table. Na and K  belong to group 1 of the periodic table.

#### Question 61:

The atomic numbers of three elements A, B and C are given below:

 Element Atomic number A 5 B 7 C 10

(i) Which element belongs to group 18?
(ii) Which element belongs to group 15?
(iii) Which element belongs to group 13?
(iv) To which period/periods do these elements belong?

(i) Element C belongs to group 18.
Electronic configuration of element C (10) = 2,8
Therefore, group number = valence shell + 10
= 8+10 = 18.

(ii) Element B belongs to group 15.
Electronic configuration of element B (15) = 2,8,5
Therefore, group number = valence shell + 10
= 5+10 = 15.

(iii) Element A belongs to group 13.
Electronic configuration of element A (5) = 2,3
Therefore, group number = valence shell + 10
= 3+10 = 13.

(iv) The elements A, B and C all belong to period 2 because all three have the same valence shell, i.e, L.

#### Question 62:

An element X belongs to 3rd period and group 2 of the periodic table. State:

(a) number of valence electrons
(b) valency
(c) metal or non-metal
(d) name of the element

(a) The element X has 2 valence electrons because it belongs to group 2.

(b) Since the number of valence electrons and the valency of group 2 elements is 2, the valency of element X is 2.

(c) Group 2 is on the left side of the periodic table which consists of metals. Therefore X is a metal.

(d) The name of the element is Magnesium (Mg), since X belongs to group 2 and the 3rd period.

#### Question 63:

The following diagram shows a part of the periodic table in which the elements are arranged according to their atomic numbers. (The letters given here not the chemical symbols of the elements):

 a 3 b 4 c 5 d 6 e 7 f 8 g 9 h 10 i 11 j 12 k 13 l 14 m 15 n 16 o 17 p 18
(i) Which element has a bigger atom, a or f?
(ii) Which element has a higher valency, k or o?
(iii) Which element is more metallic i or k?
(iv) Which element is more non-metallic, d or g?
(v) Select a latter which represents a metal of valency 2.
(vi) Select a letter which represents a non-metal of valency 2.

(i) Element a has a bigger atom. In the periodic table, the atomic radius of the element decreases on moving from left to right in a period.

(ii) has a higher valency.

Electronic configuration of k : K L M
2 8 3
Therefore, valency for k = valence electron of K = 3

Electronic configuration of o: K L M
2 8 7
Therefore, valency for o = 8 - valence electron of o
= 8 - 7 = 1

(iii) The element i is more metallic because i belongs to the left side and group 1 of the periodic table. On moving from left to right in a period, the metallic character decreases.

(iv) The element g is more non-metallic, because the non-metallic character increases on moving from left to right in a period.

(v) The elements b or j are metals with a valency of 2 as they have 2 valence electrons and are placed on the left side of periodic table.

(vi) The elements f or n are metals with a valency of 2 .

Electronic configuration of f : K L
2 6
Valency of f = 8 - valence electron
= 8 - 6 = 2

Electronic configuration of n : K L M
2 8 6
Valency of n = 8 - valence electron
= 8 - 6 = 2

#### Question 64:

An element X is in group 2 of the periodic table:
(a) What will be the formula of its chloride?
(b) what will be the formula of its oxide?

(a) An element X in group 2 has a valency of 2. We know that chlorine has 7 valence electrons. Therefore, the valency of chlorine Cl is 1, that is, (8 - 7).
Now, when one atom of X combines with two atoms of Cl, an ionic compound of the formula XCl2 is formed.

(b) We know that oxygen has 6 valence electrons and needs 2 more electrons to complete its octet. Therefore, the valency of oxygen O is 2, that is, (8 - 6). Both X and O have the same valency. When one atom of X combines with one atom of O, an ionic compound of the formula XO is formed.

#### Question 65:

An element Y is in second period and group 16 of the periodic table:

(i) Is it a metal or non-metal?
(ii) What is the number of valence electrons in its atom?
(iii) What is its valency?
(iv) What is the name of the element?
(v) What will be the formula of the compound formed by Y with sodium?

(i) The element Y is a non-metal as all the elements of group 16 are non-metals.

(ii) The number of valence electrons of the element = Group number - 10
= 16-10 = 6.
(iii) Valency of Y = 8- number of valence electrons
= 8 - 6 = 2.
(iv) The name of the element is oxygen as it is the first element in group 16.

(v) Sodium is a group 1 element with a valency of 1. The valency of Y is 2. Therefore, when two atoms of sodium (Na) combine with one atom of Y, the formula of the resulting ionic compound is Na2Y.

#### Question 66:

(a) As element X has mass number 40 and contains 21 neutrons in its atom. To which group of the periodic table does it belong?
(b) The element X forms a compound X2Y. Suggest an element that Y might be and give reasons for your choice.

(a) X belongs to Group 1. The mass number of element X is 40 and the number of neutrons in its atom is 21. In order to find out the group to which the element belongs, we need to know the atomic number of the element. Once we know the atomic number of the element, the electronic configuration and valency can be determined.
Atomic number = Mass number - Number of Neutrons.
Atomic number = 40-21 =19
The electronic configuration of X is 2,8,8,1. The number of valence electrons in X is 1. Therefore, the valency of element X is 1. We know that group 1 elements have a valency of 1.

(b) Y can be oxygen because oxygen has a valency of 2. As X is monovalent, Y has to be divalent in order to form the compound X2Y.

#### Question 67:

An element X combines with oxygen to form an oxide XO. This oxide is electrically conducting.

(a) How many electrons would be there in the outermost shell of the element X?
(b) To which group of the periodic tables does the element X belong?
(c) Write the formula of the compound formed when X reacts with chlorine.

(a) The element X is electrically conductive. Therefore, it is a metal. In the case of compound XO, both the elements are divalent, since the valency of oxygen is 2. Therefore, the valency of X should also be 2.

(b) We can conclude that X belongs to Group 2. Since the valency of the element X is 2 and it is showing electrical conductivity, therefore, it is a metal.

(c) The formula of the compound formed is XCl2, since chlorine is a group 17 element with 7 valence electrons. It has a valency of 1. Therefore, when one atom of X combines with two atoms of chlorine (Cl), an ionic compound of the formula XCl2 is formed.

#### Question 68:

An element A has an atomic number of 6. Another element B has 17 electrons in its one neutral atom.

(a) In which groups of the periodic table would you expect to find these elements?
(b) what type of bond is formed between A and B?
(c) Suggest a formula of the compound formed between A and B.

(a) Element A has an atomic number of 6, so its electronic configuration is 2,4 (having 4 valence electrons).
Therefore, the group number for element A = valence shell + 10
= 4+10 =14.
Hence, the group number for element A is 14.

Element B has 17 electrons in its neutral atom. Its electronic configuration is 2,8,7.
Therefore, the group number of element B = valence shell + 10
= 7 + 10 = 17.
Hence, the group number for element B is 17.

(b) When two non-metals A and B combine, a covalent bond is formed, since group 14 elements and group 17 elements are all non-metallic in nature.

(c) When one atom of A combines with four atoms of B, a covalent compound with the formula AB4 is formed. This is because the element A has a valency of 4 and element B has a valency of 1.

#### Question 69:

The elements A, B, C and D belong to groups 1, 2, 14 and 17 respectively of the periodic table. Which of the following pairs of elements would produce a covalent bond?

(i) A and D
(ii) C and D
(iii) A and B
(iv) B and C
(v) A and C

(iii) C and D will produce a covalent bond.

The elements C and D belong to group 14 and group 17, respectively. We know that groups 14 and 17 consist of elements that are non-metallic in nature. When two non-metals combine, sharing of electrons takes place, which further results in the formation of a covalent bond.

#### Question 70:

An element X from group 2 reacts with element Y from group 16 of the periodic table.

(a) What is the formula of the compound formed?
(b) What is the nature of bond in the compound formed?

(a) An element X from group 2 has a valency of 2. An element Y from group 16 has a valency of 2. As both the elements are divalent, an atom of X combines with an atom of Y to form a compound with the formula XY.

(b) The nature of the bond in the compound is ionic, since the element X is a metal as it is from group 2. The element Y is a non-metal as it is from group 16. When a metal combines with a non-metal, a transfer of electrons takes place and an ionic bond is formed.

#### Question 71:

A metal X is in the first group of the periodic table. What will be the formula of its oxide?

The elements in the first group of the periodic table have a valency of 1. As the metal X is in the first group, its valency is 1. We know that the valency of oxygen is 2 because it needs two electrons to complete its outer shell of 8. From this, we can conclude that two atoms of metal X will combine with one atom of oxygen to form an oxide X2O. Therefore, the formula of the oxide of metal X is X2O.

#### Question 72:

An element A from group 14 of the periodic table combines with an element B from group 16.

(i) What type of chemical bond is formed?
(ii) Give the formula of the compound formed.

(i) The resulting compound is a covalent compound since the elements of group 14 and group 16 are non-metals. Therefore, when a non-metal A from group 14 combines with a non-metal B from group 16, electrons are shared between their atoms and  a covalent bond is formed.

(ii) Element A belongs to group 14 and has 4 valence electrons, and element B belongs to group 16 and has 6 valence electrons. Therefore, when 1 atom of element A combines with 2 atoms of element B, sharing of electrons takes place and the resultant compound will have the formula AB2.

#### Question 73:

An element X from group 2 of the periodic table reacts with an element Y from group 17 to form a compound.

(a) What is the nature of the compound formed?
(b) State whether the compound formed will conduct electricity or not.
(c) Give the formula of the compound formed.
(d) What is the valency of element X?
(e) How many electrons are there in the outermost shell of an atom of element Y?

(a) The resulting compound is an ionic compound since the elements of group 2 are metals and those of group 17, non-metals. When a metal reacts with a non-metal, electrons get transferred from the metallic atoms to the non-metallic atoms, and an ionic bond is formed.

(b) Yes, it will conduct electricity because an ionic compund has formed.

(c) Element X belongs to group 2 and its valency is 2. Element Y is from group 17 and its valency is 1. So, one atom of X combines with two atoms of Y to give a compound of the formula XY2.

(d) The valency of the element X is 2 because it belongs to group 2.

(e) Group 17 elements have seven valence electrons in their atoms.
Group number = valence electron+10
Valence electron = Group number - 10
= 17-10 = 7.

#### Question 74:

The following diagram shows a part of the periodic table containing first three periods in which five elements have been represented by the letters a, b, c, d and e (which are not their chemical symbols):

 1 18
 a 2 13 14 15 16 17 b c d e
(i) Select the letter which represents an alkali metal.
(ii) Select the letter which represents a nobles gas.
(iii) Select the letter which represents a halogen.
(iv) What type of bond is formed between a and e?
(v) What type  of bond is formed between d and e?

(i) The letter 'd' represents an alkali metal. This is because group 1 elements of the periodic table, except hydrogen, are all alkali metals having 1 valence electron.

(ii) The letter 'c' represents a noble gas. All the group 18 elements are noble gases or inert gases whose valence shells are completely filled with electrons.

(iii) The letter 'e' represents a halogen. Group 17 elements are known as halogens and they have 7 valence electrons.

(iv) A covalent bond is formed between 'a' and 'e'. 'a', the top most element represents hydrogen which is the only non-metal in group 1. Therefore, when a non-metal 'a' forms a compound with another non-metal 'e', sharing of electrons takes place resulting in the formation of a covalent bond.

(v) An ionic bond is formed between 'd' and 'e'. When a metal reacts with a non-metal, a transfer of electrons takes place from the metal to the non-metal and results in the formation of an ionic bond.

#### Question 75:

The elements A, B and C belong to groups 1, 14, and 17 respectively of the periodic table.
(a) Which two elements will form a covalent compound?
(b) Which two elements will form an ionic compound?

(a) The element B combines with the element C to form a covalent compound. The elements B and C belong to group 14 and group 17, respectively, which are non-metals. Therefore, when two non-metals react with each other, sharing of electrons takes place, which further results in the formation of a covalent bond.

(b) The element A combines with the element C to form an ionic compound. The element A belonging to group 1 is a metal. The element C belonging to group 17 is a non-metal. Therefore, when a metal combines with a non-metal, electrons are transferred from the metal to the non-metal, resulting in the formation of an ionic bond.

#### Question 76:

Find the neutral atom in the periodic table which has the same number of electrons as K+ and Cl. What is this number?

Argon is a neutral atom in the periodic table and has the same number of electrons as K+ and Cl-. The atomic number of argon is 18. It has 18 electrons. Potassium (K) has 19 electrons but because it loses one electron, it becomes K+ and has 18 electrons. On the other hand, chlorine (Cl)  has 17 electrons but because it gains one electron, it forms Cl- and has 18 electrons.

#### Question 77:

Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electron shells but different number of electrons in their outermost shells. It was found that elements A and G combine to form an ionic compound. This ionic compound is added in a small amount to almost all vegetables and dishes during cooking. Oxides of elements A and B are basic in nature while those of elements E and F are acidic. The oxide of element D is, however, almost neutral. Based on the above information, answer the following questions:

(a) To which group or period of the periodic table do these elements belong?
(b) What would be the nature of compound formed by a combination of elements B and F?
(c) Which two of these elements could definitely be metals?
(d) Which one of the eight elements is most likely to be found in gaseous state at room temperature?
(e) If the number of electrons in the outermost shell of elements C and G be 3 and 7 respectively, write the formula of the compound formed by the combination of C and G.

(a) The elements belong to the third period of the periodic table, and have the same number of electron shells but different number of electrons in their outermost shells. The number of valence electrons in these elements increases from 1 to 8, on moving from left to right in this period.

(b) When metal B combines with a non-metal F, an ionic compound is formed due to the transfer of electrons from the metal to the non-metal. This is because in the third period, elements A, B and C are metals, D is a metalliod, and E, F, G and H are non-metals.

(c) A and B are definitely metals because in the 3rd period, at the extreme left, only metals are placed in the periodic table .

(d) The element H is most likely to be found in a gaseous state at room temperature. This is because the last group elements in the periodic table are all in a gaseous state at room tempearture.

(e) If the number of valence electrons of element C is 3 and those of element G is 7, then 1 atom of element C will combine with 3 atoms of element G to form a compound with the formula CG3.

#### Question 78:

Write the names and symbols of two very reactive metals belonging to group 1 of the periodic table. Explain by drawing electronic structure, how either one of the two metals reacts with a halogen. With which name is the bond formed between these elements known and what is the class of the compound so formed known? State any four physical properties of such compounds.

Sodium (Na) and Potassium (K) are the two very reactive metals belonging to group 1 of the periodic table.
Let us see the formation of an ionic compound, namely Sodium Fluoride (NaF), when Sodium (Group 1 element) reacts with Fluorine ( Group 17 element). Sodium has 1 electron in its outermost shell whereas Fluorine has 7 electrons. When Sodium and Fluorine react, a transfer of electrons takes place from Sodium to Fluorine and an ionic compound is formed.
The following drawing clearly explains the reaction of Sodium and Fluorine:

Halogens are the non-metal elements which belong to group 17 of the periodic table. Metals are electropositive and have a tendency to lose electrons while non-metals are electronegative and have a tendency to accept electrons. Therefore, when a metal reacts with a non-metal, a transfer of electrons takes place, resulting in the formation of an ionic bond. The resulting compound is called an ionic compound.
Physical properties of Ionic Compounds:
1. Ionic compounds are solids.
2. They have high melting and boiling points.
3. They are generally soluble in water.
4. They conduct electricity in molten state.

#### Question 79:

The non-metal A is an important constituent of our food and most of the fuels around us. A forms two oxides B and C. The oxide B is poisonous whereas oxide C causes global warming.

(a) Identify A, B and C.
(b) To which group of periodic table does A belong?
(c) Name another element which is placed in the  same group as A.

(a) The non-metal A which is an important constituent of our food and most of the fuels around us is carbon. Carbon combines with oxygen in two ways to form two oxides. When one atom of carbon combines with one atom of oxygen, carbon monoxide (CO), a poisonous gas is formed. And when one atom of carbon combines with two atoms of oxygen, carbon dioxide (CO2) is formed which causes global warming.

(b) Carbon belongs to group 14 of the periodic table.

(c) Silicon (Si) is another element which is placed in the same group 14 of the periodic table.

#### Question 80:

A non-metal X which is the largest constituent of air combines with hydrogen when heated in the presence of iron as catalyst to form a gas Y. When gas Y is treated with sulphuric acid, it forms a compound Z which is used as a chemical fertiliser.

(a) What are X, Y and Z?
(b) To which group of periodic table does X belong?
(c) Name the period of periodic table in which X is placed.
(d) Which element is placed just before X in the period?
(e) Which element is placed just after X in the period?

(a) Element X is Nitrogen gas (N2) since the largest constituent of air is nitrogen. When nitrogen reacts with hydrogen in the presence of iron as a catalyst, ammonia (NH3) is formed. Therefore, Y is ammonia (NH3). When ammonia reacts with sulpuric acid, ammonium sulphate is formed which is used as a chemical fertilizer. Therefore, Z is ammonium sulphate (NH4)2SO4.

(b) Nitrogen belongs to group 15 of the periodic table.

(c) Nitrogen is placed in the second period of the periodic table.

(d) Carbon (C) is placed just before nitrogen in the period.

(e) Oxygen (O) is placed just after nitrogen in the period.

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