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Page No 38:
Question 1:
Upto which element, the Law of Octaves was found to be applicable
(a) Oxygen
(b) Calcium
(c) Cobalt
(d) Potassium
Answer:
Law of octaves was found to be applicable up to calcium.
Hence, the correct answer is option (b).
Page No 38:
Question 2:
According to Mendeléev's Periodic Law, the elements were arranged in the periodic table in the order of
(a) increasing atomic number
(b) decreasing atomic number
(c) increasing atomic masses
(d) decreasing atomic masses
Answer:
According to Mendeléev's Periodic Law, the elements were arranged in the periodic table in the order of their increasing atomic mass.
Hence, the correct answer is option (c).
Page No 38:
Question 3:
In Mendeléev’s Periodic Table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the perioidc table later
(a) Germanium
(b) Chlorine
(c) Oxygen
(d) Silicon
Answer:
Out of the given options, only germanium was found later.
Hence, the correct answer is option (a).
Page No 38:
Question 4:
Which of the following statement (s) about the Modern Periodic Table are incorrect
(i) The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number
(ii) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses
(iii) Isotopes are placed in adjoining group (s) in the Periodic Table
(iv) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number
(a) (i) only
(b) (i), (ii) and (iii)
(c) (i), (ii) and (iv)
(d) (iv) only
Answer:
The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number and not on the basis of their increasing atomic masses. Also, the Isotopes are not placed in the adjoining group (s) in the Periodic Table.
Hence, the correct answer is option (b).
Page No 39:
Question 5:
Which of the following statements 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
Answer:
The modern periodic table has 18 vertical columns known as Groups.
Hence, the correct answer is option (c).
Page No 39:
Question 6:
Which of the given elements A, B, C, D and E with atomic number 2, 3, 7, 10 and 30 respectively belong to the same period?
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E
Answer:
Electronic configuration of A = 2
Electronic configuration of B = 2,1
Electronic configuration of C = 2,5
Electronic configuration of D = 2,8
Electronic configuration of E = 2,8,8,8,4
If the electronic configuration of the given elements is written, only elements B, C and D have electrons occupying only two shells. So, only elements B, C and D would be placed in the same period in the modern periodic table.
Hence, the correct answer is option (b).
Page No 39:
Question 7:
The elements A, B, C, D and E have atomic number 9, 11, 17, 12 and 13 respectively. Which pair of elements belong to the same group?
(a) A and B
(b) B and D
(c) A and C
(d) D and E
Answer:
Electronic configuration of A = 2,7
Electronic configuration of B = 2,8,1
Electronic configuration of C = 2,8,7
Electronic configuration of D = 2,8,2
Electronic configuration of E = 2,8,3
According to the electronic configuration of the given elements, only elements A and C have 7 valance electrons. So, A and C belong to the same group in the modern periodic table.
Hence, the correct answer is option (c).
Page No 39:
Question 8:
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
Answer:
According to the electronic configuration of the element, it has electrons occupying the first two shells so, it belongs to the second period. As the number of valance electrons is 8, so this element belongs to the eighteenth group in the modern periodic table.
Hence, the correct answer is option (c).
Page No 39:
Question 9:
An element which is an essential constituent of all organic compounds belongs to
(a) group 1
(b) group 14
(c) group 15
(d) group 16
Answer:
An element which is an essential constituent of all organic compounds belongs to is carbon, it is located in the second period and group number 14 in the periodic table.
Hence, the correct answer is option (c).
Page No 39:
Question 10:
Which of the following is the outermost shell for elements of period 2?
(a) K shell
(b) L shell
(c) M shell
(d) N shell
Answer:
For all the elements in period two, the outer most shell is the second shell which is named as the L shell.
Hence, the correct answer is option (b).
Page No 40:
Question 11:
Which one of the following elements exhibit maximum number of valence electrons?
(a) Na
(b) Al
(c) Si
(d) P
Answer:
Out of the given options, phosphorus has an atomic number of 15, that means it has 15 electrons and thus it has the highest number of electrons.
Hence, the correct answer is option (d).
Page No 40:
Question 12:
Which of the following gives the correct increasing order of the atomic radii of O, F and N ?
(a) O, F, N
(b) N, F, O
(c) O, N, F
(d) F, O, N
Answer:
In the modern periodic table, the atomic radius decreases from left to right in the period. So, element fluorine has the smallest atomic radius while nitrogen has the highest among the given options.
Hence, the correct answer is option (d).
Page No 40:
Question 13:
Which among the following elements has the largest atomic radii?
(a) Na
(b) Mg
(c) K
(d) Ca
Answer:
In the modern periodic table, the atomic radius increases down the group and decreases along the period from left to right. So, out of the given options sodium has the smallest atomic radius while calcium would have the largest atomic radius.
Hence, the correct answer is option (d).
Page No 40:
Question 14:
Which of the following elements would lose an electron easily?
(a) Mg
(b) Na
(c) K
(d) Ca
Answer:
In the modern periodic table, the metallic character of the elements increases down the group and the metallic character decreases along the period from left to right.
Hence, the correct answer is option (c).
Page No 40:
Question 15:
Which of the following elements does not lose an electron easily?
(a) Na
(b) F
(c) Mg
(d) Al
Answer:
In the modern periodic table, the electronegativity of the elements decreases down the group and increases along the period from left to right.
Hence, the correct answer is option (b).
Page No 40:
Question 16:
Which of the following are the characteristics of isotopes of an element?
(i) Isotopes of an element have same atomic masses
(ii) Isotopes of an element have same atomic number
(iii) Isotopes of an element show same physical properties
(iv) Isotopes of an element show same chemical properties
(a) (i), (iii) and (iv)
(b) (ii), (iii) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer:
Isotopes of an element have the same atomic number but different atomic masses. They have similar chemical and physical properties.
Hence, the correct answer is option (b).
Page No 41:
Question 17:
Arrange the following elements in the order of their decreasing metallic character
Na, Si, Cl, Mg, Al
(a) Cl > Si >Al > Mg >Na
(b) Na >Mg >Al >Si > Cl
(c) Na > Al > Mg > Cl > Si
(d) Al > Na> Si > Ca> Mg
Answer:
In the modern periodic table, The metallic character of the elements increases down the group and the metallic character decreases along the period from left to right.
hence, the correct answer is option (b).
Page No 41:
Question 18:
Arrange the following elements in the order of their increasing nonmetallic character
Li, O, C, Be, F
(a) F < O < C < Be < Li
(b) Li < Be < C < O< F
(c) F < O < C < Be < Li
(d) F < O < Be < C < Li
Answer:
In the modern periodic table, the non-metallic character of the elements decreases down the group and it increases along the period from left to right.
Hence, the correct answer is option (b).
Page No 41:
Question 19:
What type of oxide would Eka– aluminium form?
(a) EO3
(b) E3 O2
(c) E2 O3
(d) EO
Answer:
Eka-aluminium is the element gallium, it has a valency of 3. When gallium forms an oxide it would have a formula of E2 O3.
Hence, the correct answer is option (c).
Page No 41:
Question 20:
Three elements B, Si and Ge are
(a) metals
(b) non-metals
(c) metalloids
(d) metal, non-metal and metalloid respectively
Answer:
Out of the given options, all the elements come under the category fo metalloids.
Hence, the correct answer is option (c).
Page No 41:
Question 21:
Which of the following elements will form an acidic oxide?
(a) An element with atomic number 7
(b) An element with atomic number 3
(c) An element with atomic number 12
(d) An element with atomic number 19
Answer:
Non-metallic oxides are acidic in nature. Out of the given options, only the element with atomic number 7, i.e., nitrogen is a non-metal.
Hence, the correct answer is option (a).
Page No 41:
Question 22:
The element with atomic number 14 is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?
(a) Metal
(b) Metalloid
(c) Non-metal
(d) Left-hand side element
Answer:
As the atomic number of the given element is 14 so, the element is silicon. Element silicon is a metalloid.
Hence, the correct answer is option (b).
Page No 42:
Question 23:
Which one of the following depict the correct representation of atomic radius(r) of an atom?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Answer:
The atomic radius(r) of an atom is the distance between the centre of the nucleus and the outermost shell of an isolated atom. In representation (ii), the outermost shell is K and atomic radius is correctly depicted from centre of nucleus to shell K while in representation (iii), the outermost shell is M and atomic radius is correctly depicted from centre of nucleus to shell M.
Hence, the correct answer is option (b).
Page No 42:
Question 24:
Which one of the following does not increase while moving down the group of the periodic table?
(a) Atomic radius
(b) Metallic character
(c) Valence
(d) Number of shells in an element
Answer:
In the modern periodic table, on moving down the group the valency of the elements does not increase.
Hence, the correct answer is option (c).
Page No 42:
Question 25:
On moving from left to right in a period in the periodic table, the size of the atom.
(a) increases
(b) decreases
(c) does not change appreciably
(d) first decreases and then increases
Answer:
On moving from left to right in a period in the periodic table, the size of the atom decreases.
Hence, the correct answer is option (b).
Page No 43:
Question 26:
Which of the following set of elements is written in order of their increasing metallic character?
(a) Be Mg Ca
(b) Na Li K
(c) Mg Al Si
(d) C O N
Answer:
In the modern periodic table, metallic character of elements increases down the group and the metallic character decreases along the period from left to right.
Hence, the correct answer is option (a).
Page No 43:
Question 27:
The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as? Give one example of such a set of elements.
Answer:
Such an arrangement of elements is called triads. Example: lithium sodium and potassium make a triad. Their atomic masses are 6.9, 23.0 and 39.0 respectively. An average mass of Li and K is approximately equal to the atomic mass of Na.
Page No 43:
Question 28:
Elements have been arranged in the following sequence on the basis of their increasing atomic masses.
| F, | Na, | Mg, | Al, | Si, | P, | S, | Cl, | Ar, | K |
(a) Pick two sets of elements which have similar properties.
(b) The given sequence represents which law of classification of elements?
Answer:
(a) Sodium, magnesium aluminium and potassium are all metals while fluorine, phosphorus, sulphur, chlorine and argon are all non-metals.
(b) The given sequence represents Mendeleev's law of periodicity.
Page No 43:
Question 29:
Can the following groups of elements be classified as Döbereiner's triad?
(a) Na, Si, Cl
(b) Be, Mg, Ca
Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40 Explain by giving reason.
Answer:
(a) For set a, even though silicon's atomic mass is similar to the arithmetic mean of the atomic masses of sodium and chlorine. The physical properties of the elements are not similar. Hence, sodium, silicon and chlorine are not dobereiner's triad.
(b) For set b, if the arithmetic mean of the atomic masses of beryllium and calcium is taken it is similar to the atomic mass of magnesium and the physical properties of the three elements are similar. So, Beryllium, magnesium and calcium can be classified as dobereiner's triad.
Page No 43:
Question 30:
In Mendeléev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give reason for the same.
Answer:
In Mendeleev′s periodic table there are instances where elements with higher atomic masses are placed before the element with lower atomic mass. This was done to ensure that elements with similar properties were included in the same group. Hence Cobalt was placed before Nickel despite the higher atomic number of Cobalt than Nickel.
Page No 43:
Question 31:
“Hydrogen occupies a unique position in Modern Periodic Table”. Justify the statement.
Answer:
Hydrogen occupies a unique position in the modern periodic table because of the following reasons:
1. Electronic configuration of hydrogen is similar to that of alkali metals as both have one electron in the outermost shell.
2. Properties of hydrogen is similar to that of the halogen elements as both have the same tendency of accepting electrons to complete their octet electronic configuration.
Page No 43:
Question 32:
Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeléev.
Answer:
In the Mendeleev's periodic table Eka-silicon and Eka-aluminium later know to be germanium and gallium. Chlorides of germanium and gallium have the formula of and .
Page No 43:
Question 33:
Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.
Answer:
Element A has 3 valance electrons so, its valency is 3 and it belongs to group 13 in the modern periodic table.
Element B has 4 valance electrons so, its valency is 4 and it belongs to group 14 in the modern periodic table.
Element C has 2 valance electrons so, its valency is 2 and it belongs to group 2 ​in the modern periodic table.
Page No 44:
Question 34:
If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?
Answer:
Elements in group 14 have a valency of 4 hence, the formula of its chloride would be and the bonding in the compound would be of covalent in nature.
Page No 44:
Question 35:
Compare the radii of two species X and Y. Give reasons for your answer.
(a) X has 12 protons and 12 electrons
(b) Y has 12 protons and 10 electrons
Answer:
In the two given options, species X and Y represents the same element magnesium. X is magnesium atom and Y is magnesium ion. Out of Mg and Mg2+ ion Mg atom would be larger as the number of energy shells in Magnesium atom is three while the number of energy shells in magnesium ion is two.
Page No 44:
Question 36:
Arrange the following elements in increasing order of their atomic radii.
(a) Li, Be, F, N
(b) Cl, At, Br I
Answer:
(a) The atomic size decreases on moving left to right in a period.
F < N < Be < Li
(b) The atomic size increases down the group.
Cl < Br < I < At
Page No 44:
Question 37:
Identify and name the metals out of the following elements whose electronic configurations are given below.
(a) 2, 8, 2
(b) 2, 8, 1
(c) 2, 8, 7
(d) 2, 1
Answer:
Out of the given options, if the electronic configuration of a, b, c and d are considered, they represent the elements magnesium, sodium, chlorine and lithium respectively. Out of these elements magnesium, sodium and lithium are metals.
Page No 44:
Question 38:
Write the formula of the product formed when the element A (atomic number 19) combines with the element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed?
Answer:
Element A has atomic number 19 so, it is potassium. Element B has atomic number 17 so, it is chlorine. As the valency of potassium is 1 and the valency of chlorine is also 1, the formula of the compound formed would be KCl or AB.
The nature of the bond between potassium and chlorine in KCl is an ionic bond.
Page No 44:
Question 39:
Arrange the following elements in the increasing order of their metallic character
| Mg, | Ca, | K, | Ge, | Ga |
Answer:
In the modern periodic table metallic character of the elements increases down the group and the metallic character decreases along the period from left to right.
The position of the given elements in the periodic table are:
Mg - 2nd group and 3rd period
Ca - 2nd group and 4th period
K - 1st group and 4th period
Ge - 14th group and 4th period
Ga - 13th group and 4th period
According to the positions of these elements in the modern periodic table, Ge has the lowest metallic character and K has the highest metallic character. The arrangements of these metals in the increasing order of their metallic character would be Ge < Ga < Mg < Ca < K.
Page No 44:
Question 40:
Identify the elements with the following property and arrange them in increasing order of their reactivity
(a) An element which is a soft and reactive metal
(b) The metal which is an important constituent of limestone
(c) The metal which exists in liquid state at room temperature
Answer:
(a) An element which is a soft and reactive metal is sodium.
(b) The metal which is an important constituent of limestone is calcium.
(c) The metal which exists in the liquid state at room temperature is mercury.
The arrangement of these elements in increasing order of their reactivity would be mercury (Hg) < calcium (Ca) < sodium (Na).
Page No 44:
Question 41:
Properties of the elements are given below. Where would you locate the following elements in the periodic table?
(a) A soft metal stored under kerosene
(b) An element with variable (more than one) valency stored under water.
(c) An element which is tetravalent and forms the basis of organic chemistry
(d) An element which is an inert gas with atomic number 2
(e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of “ anodising”
Answer:
(a) A soft metal stored under kerosene is sodium. Sodium is located at 1st group and 3rd period in the periodic table.
(b) An element with variable (more than one) valency stored underwater is phosphorus, it is located at 15th group and 3rd period in the periodic table.
(c) An element which is tetravalent and forms the basis of organic chemistry is carbon, it is located at 14th group and 2nd period in the periodic table.
(d) An element which is an inert gas with atomic number 2 is helium, it is located at 18th group and 1st period in the periodic table.
(e) An element whose thin oxide layer is used to make other elements corrosion-resistant by the process of “ anodising” is aluminium, it is located at 13th group and 3rd period in the periodic table.
Page No 45:
Question 42:
An element is placed in 2nd Group and 3rd Period of the Periodic Table, burns in presence of oxygen to form a basic oxide.
(a) Identify the element
(b) Write the electronic configuration
(c) Write the balanced equation when it burns in the presence of air
(d) Write a balanced equation when this oxide is dissolved in water
(e) Draw the electron dot structure for the formation of this oxide
Answer:
(a) The element is magnesium (Mg).
(b) Electronic configuration of the element is 2, 8, 2.
(c) When magnesium burns in the air it forms magnesium oxide.
(d) When magnesium oxide is dissolved in water it forms magnesium hydroxide.
(e)
Page No 45:
Question 43:
An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide.
(a) Where in the periodic table are elements X and Y placed?
(b) Classify X and Y as metal (s), non-metal (s) or metalloid (s)
(c) What will be the nature of oxide of element Y? Identify the nature of bonding in the compound formed
(d) Draw the electron dot structure of the divalent halide
Answer:
(a) Element X is chlorine, it is placed at 17th group and 3rd period in the periodic table. Element Y is calcium, it is placed at the 2nd group and 4th period in the periodic table.
(b) Element X is a non-metal and element Y is a metal.
(c) Element Y is metal and thus, the oxide of this metal will be basic in nature. The chemical formula for calcium oxide is and the bonding in the compound is ionic in nature.
(d)
Page No 45:
Question 44:
Atomic number of a few elements are given below
10, 20, 7, 14
(a) Identify the elements
(b) Identify the Group number of these elements in the Periodic Table
(c) Identify the Periods of these elements in the Periodic Table
(d) What would be the electronic configuration for each of these elements?
(e) Determine the valency of these elements
Answer:
(a) The given elements are neon, calcium, nitrogen and silicon.
(b) Neon - group number 18, calcium - group number 2, nitrogen - group number 15 and silicon - group number 14.
(c) Neon - 2nd period, calcium - 4th period, nitrogen - 2nd period and silicon - 3rd period.
(d) Neon - 2, 8
Calcium - 2, 8, 8, 2
nitrogen - 2, 7
silicon - 2, 8, 4
(e) Neon - valency is 0, calcium - valency is 2, nitrogen - valency is 3, silicon - valency is 4.
Page No 45:
Question 45:
Complete the following cross word puzzle (Figure 5.1)
Across:
(1) An element with atomic number 12.
(3) Metal used in making cans and member of Group 14.
(4) A lustrous non-metal which has 7 electrons in its outermost shell.
Down:
(2) Highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene.
(5) The first element of second Period
(6) An element which is used in making fluorescent bulbs and is second member of Group 18 in the Modern Periodic Table
(7) A radioactive element which is the last member of halogen family.
(8) Metal which is an important constituent of steel and forms rust when exposed to moist air.
(9) The first metalloid in Modern Periodic Table whose fibres are used in making bullet-proof vests
| 1 | 7 | 2 | |||||||||
| 3 | 8 | 9 | 5 | ||||||||
| 4 | 6 | ||||||||||
Answer:
Across:
(1) Magnesium
(3) Tin
(4) Iodine
Down:
(2) Sodium
(5) Lithium
(6) Neon
(7) Astatine
(8) Iron
(9) Boron
Page No 46:
Question 46:
(a) In this ladder (Figure 5.2) symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table.
(b) Arrange them in the order of their group also.
Answer:
(a) H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca
(b) Group 1 — H, Li, Na, K
Group 2 — Be, Mg, Ca
Group 13 — B, Al
Group 14 — C, Si
Group 15 — N, P
Group 16 — O, S
Group 17 — F, Cl
Group 18 — He, Ne, Ar
Page No 46:
Question 47:
Mendeléev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.
(a) Name the elements which have taken the place of these elements
(b) Mention the group and the period of these elements in the Modern Periodic Table.
(c) Classify these elements as metals, non-metals or metalloids
(d) How many valence electrons are present in each one of them?
Answer:
(a) Eka-Silicon was replaced by germanium, Eka-aluminium was replaced by gallium in the Mendeleev's periodic table.
(b) Germanium-group number 14 and 4th period.
Gallium-group number 13 and 4th period.
(c) Germanium is a metalloid and gallium is a metal.
(d) Germanium has 4 electrons and gallium has 3 valence electrons.
Page No 46:
Question 48:
(i) Electropositive nature of the element(s) increases down the group and decreases across the period
(ii) Electronegativity of the element decreases down the group and increases across the period
(iii) Atomic size increases down the group and decreases across a period (left to right)
(iv) Metallic character increases down the group and decreases across a period.
On the basis of the above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9.
(a) Name the most electropositive element among them
(b) Name the most electronegative element
(c) Name the element with smallest atomic size
(d) Name the element which is a metalloid
(e) Name the element which shows maximum valency.
Answer:
(a) Lithium is the most electropositive element.
(b) Fluorine is the most electronegative element.
(c) Fluorine is the element with the smallest atomic size.
(d) Boron is a metalloid.
(e) Carbon is the element which shows maximum valency.
Page No 47:
Question 49:
An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants.
(a) Identify the element X
(b) Write the electronic configuration of X
(c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals?
(d) What would be the nature (acidic/ basic) of oxides formed?
(e) Locate the position of the element in the Modern Periodic Table.
Answer:
(a) Element X is sulphur.
(b) Electronic configuration of X is 2, 8, 6.
(c)
(d) Oxides of sulphurs are acidic in nature.
(e) Sulphur is placed at group number 16th and 3rd period in the modern periodic table.
Page No 47:
Question 50:
An element X of group 15 exists as diatomic molecule and combines with hydrogen at 773 K in presence of the catalyst to form a compound, ammonia which has a characteristic pungent smell.
(a) Identify the element X. How many valence electrons does it have?
(b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?
(c) Draw the electron dot structure for ammonia and what type of bond is formed in it?
Answer:
(a) Element X is nitrogen. Nitrogen has 5 valence electrons.
(b)
Nitrogen molecule has covalent bonds in between the nitrogen atoms.
(c)
Ammonia molecule ahs covalent bonds between the nitrogen and hydrogen atoms.
Page No 47:
Question 51:
Which group of elements could be placed in Mendeléev's Periodic Table without disturbing the original order? Give reason.
Answer:
Noble gases could be placed in Mendeléev's Periodic Table without disturbing the original order.
According to Mendeleev’s classification, the properties of elements are the periodic function of their atomic masses and there is a periodic recurrence of elements with similar physical and chemical properties.
Noble gases like helium (He), neon (Ne) and argon (Ar) have been mentioned in many a context before Mendeleev's periodic table was discovered. These gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. One of the strengths of Mendeléev’s periodic table was that, when the noble gases were discovered, they could be placed in a new group named zero group after the 8th group without disturbing the existing order.
Page No 47:
Question 52:
Give an account of the process adopted by Mendeléev for the classification of elements. How did he arrive at “Periodic Law”?
Answer:
When Mendeléev started his work, 63 elements were known. He examined the relationship between the atomic masses of the elements and their physical and chemical properties. Among chemical properties, Mendeléev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.
He took 63 cards and on each card, he wrote down the properties of one element. He sorted out the elements with similar properties and pinned the cards together on a wall. He observed that most of the elements got a place in a periodic table and were arranged in the order of their increasing atomic masses. It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. On this basis, Mendeléev formulated a periodic law, which states that ‘the properties of elements are the periodic function of their atomic masses’.
Further, Mendeléev left some gaps in his Periodic Table. Mendeléev boldly predicted the existence of some elements that had not been discovered at that time. Mendeléev named them by prefixing a sanskrit numeral, Eka (one) to the name of preceding element in the same group. For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.
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