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#### Question 1:

Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2, 2

The atomic number of magnesium is 12 which means a magnesium atom has 12 electrons. So, the electronic configuration can be written as  2, 8, 2.

Hence, the correct answer is option (b).

#### Question 2:

Rutherford’s 'alpha (α) particles scattering experiment' resulted in to discovery of
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass

During Rutherford’s α-particle scattering experiment, some of the alpha particles are deflected through small and large angles. This observation shows that there is a 'centre of positive charge' in the atom which repels the positively charged alpha particles and deflects them from their original path. This centre of positive charge in the atom is known as the nucleus. Therefore, Rutherford’s α-particle scattering experiment led to the discovery of the atomic nucleus.

Hence, the correct answer is option (c).

#### Question 3:

The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

(a) ${}_{15}{}^{31}\mathrm{X}$

(b) ${}_{16}{}^{31}\mathrm{X}$

(c) ${}_{15}{}^{16}\mathrm{X}$

(d) ${}_{16}{}^{15}\mathrm{X}$

The correct representation of an element is given by ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$
where,
A is mass number of the element
X is symbol of the element
Z is the atomic number of the element

For the given element X,
Atomic number = Number of electrons = 15
Mass number = 15 + 16 = 31

Thus, the representation of element X is ${}_{15}{}^{31}\mathrm{X}$

Hence, the correct answer is option (a).

#### Question 4:

Dalton’s atomic theory successfully explained
(i) Law of conservation of mass
(ii) Law of constant composition
(iv) Law of multiple proportion
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)

Dalton’s atomic theory successfully explained the law of conservation of mass, law of constant composition, and law of multiple proportions.

Hence, the correct answer is option (d).

#### Question 5:

Which of the following statements about Rutherford’s model of atom are correct?
(i) considered the nucleus as positively charged
(ii) established that the α–particles are four times as heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson’s model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) only (i)

Rutherford's model of atom explained the presence of a nucleus in the centre and electrons around the nucleus revolving in round orbitals like the solar system. The positively charged alpha particles were deflected by the nucleus. This shows the nucleus is positively charged.

Hence, the correct answer is option (a).

#### Question 6:

Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)

The formula for the atomic number and mass number can be written as:
Atomic number = number of protons = number of electrons
Mass number = Number of protons + Number of neutrons

Hence, the correct answer is option (d).

#### Question 7:

In the Thomson’s model of atom, which of the following statements are correct?
(i) the mass of the atom is assumed to be uniformly distributed over the atom
(ii) the positive charge is assumed to be uniformly distributed over the atom
(iii) the electrons are uniformly distributed in the positively charged sphere
(iv) the electrons attract each other to stabilise the atom
(a) (i), (ii) and (iii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)

Thomson's model could be compared with a raisin pudding model according to which the mass of the atom is uniformly distributed over the atom in the form of positive charge and electrons are uniformly distributed over the atom. Electrons do not attract each other.
Hence, the correct answer is option (a).

#### Question 8:

Rutherford’s $\mathrm{\alpha }$-particle scattering experiment showed that
(i) electrons have negative charge
(ii) the mass and positive charge of the atom is concentrated in the nucleus
(iii) neutron exists in the nucleus
(iv) most of the space in atom is empty

Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv)

The following conclusions were drawn by Rutherford from $\mathrm{\alpha }$-particle scattering experiment:
(i) Most of the space inside the atom is empty.
(ii) All the positive charge and mass of the atom were concentrated in a very small volume within the atom.
(iii) The positive charge of the atom occupies very little space.

Hence, the correct answer is option (b).

#### Question 9:

The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
(a) 13
(b) 10
(c) 14
(d) 16

Mass number of the atom = 27
Number of neutrons = 14
Number of protons = 27 − 14 = 13
Number of electrons in the atom = 13
Number of electrons in ion with 3 positive charges = 13 − 3 = 10

Hence, the correct answer is option (b).

#### Question 10:

Identify the Mg2+ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons respectively Fig.4.1

A magnesium atom can be represented as :  ${}_{12}{}^{24}\mathrm{Mg}$
For magnesium atom,
Atomic number = Number of  protons = Number of electrons = 12
Number of neutrons = Mass number − Number of protons = 24 − 12 = 12
If magnesium loses 2 electrons, it forms Mg2+ ion.
Number of electrons in Mg2+= 10

Hence, the correct answer is option (d).

#### Question 11:

In a sample of ethyl ethanoate (CH3COOC2H5) the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars.

The isotope of an element contains the same number of electrons and protons but a different mass number. As the number of electrons and number of protons are equal in an atom, a different mass number means a different number of neutrons.

Hence, the correct answer is option (c).

#### Question 12:

Elements with valency 1 are
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals

The metals which have 1 valence electron and non-metals which have 7 valence electrons can exhibit a valency of 1. Therefore, both metal and non-metals can show a valency of 1.

Hence, the correct answer is option (c).

#### Question 13:

The first model of an atom was given by
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson

J.J. Thomson was the first one to propose a model for the structure of an atom.

Hence, the correct answer is option (d).

#### Question 14:

An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4

For the atom
Number of neutrons = 4
Number of protons = 3
Therefore, the number of electrons = 3
Hence, the distribution of electrons = 2,1
Since the atom can lose 1 electron to achieve the inert gas electron configuration, therefore, the valency of the given atom is 1.

Hence, the correct answer is option (c).

#### Question 15:

The electron distribution in an aluminium atom is
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8

Aluminium has 13 electrons. Its electron distribution can be written as 2, 8, 3.

Hence, the correct answer is option (a).

#### Question 16:

Which of the following in Fig. 4.2 do not represent Bohr’s model of an atom correctly? (a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)

According to Bohr's model of an atom, the first shell can have a maximum of 2 electrons and the second shell can have a maximum of 8 electrons. Fig. (ii) contains 4 electrons in K shell, and fig. (iv) contains 9 electrons in L shell, which are not in accordance with Bohr's model.

Hence, the correct answer is option (c).

#### Question 17:

Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.

An atom has an equal number of electrons and protons since it is electrically neutral.

Hence, the correct answer is option (a).

#### Question 18:

Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order
(i) Rutherford’s atomic model
(ii) Thomson’s atomic model
(iii) Bohr’s atomic model
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)

The correct chronological order of the improvements in atomic models is Thomson's atomic model, followed by Rutherford's atomic model which is followed by Bohr's atomic model.

Hence, the correct answer is option (c).

#### Question 19:

Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.

Yes, it is possible. Hydrogen has one electron, one proton and no neutron.

#### Question 20:

Write any two observations which support the fact that atoms are divisible.

Two observations which support the fact that atoms are divisible are:
(i) Ionic compounds are formed because of the formation of ions that involves the transfer of electrons.
(ii) The presence of isotopes for the same element is possible due to the difference in the number of neutrons.
The above observations show that an atom is formed by different particles such as electrons, protons, and neutrons, i.e. atom is divisible.

#### Question 21:

No,  35Cl and 37Cl have the same valencies. They are isotopes of chlorine and have the same number of protons and electrons. They have same electron distribution, therefore, the same valency.

#### Question 22:

Why did Rutherford select a gold foil in his α–ray scattering experiment?

Gold is a heavy metal with a high mass number and hence the fast-moving alpha particles will not be able to push the atoms and will scatter after colliding with the gold atoms. Also, gold is a highly malleable element can be beaten into very thin sheets.

#### Question 23:

Find out the valency of the atoms represented by the Fig. 4.3 (a) and (b). (a) The electronic configuration of the atom is 2,8,8. Its outermost shell has a complete octet. Hence, its valency is zero.
(b) The electronic configuration of the atom is 2,7. Its outermost shell has an incomplete octet. It needs to gain one electron to complete its octet. Hence, its valency is 1.

#### Question 24:

One electron is present in the outer most shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outer most shell?

An element X is a metal because one electron is present in the outermost shell. When this valence electron is removed from the outermost shell, a cation (positive ion) will be formed with a charge of +1.

#### Question 25:

Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17).

The atomic number of chlorine is 17. The electronic distribution of chlorine is 2, 8, 7. Hence it has 2 electrons in its innermost shell, 8 electrons in its second shell, and 7 electrons in the outermost shell respectively. Therefore, the L shell of chlorine contains 8 electrons.

#### Question 26:

In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed?

An atom having 6 electrons in its outermost shell will require two electrons to complete the octet.

The ion formed will have a negative charge of −2.

#### Question 27:

What information do you get from the Fig. 4.4 about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form. Element Atomic Number Mass Number Valency X 5 11 3 Y 8 18 2 Z 15 31 3, 5

#### Question 28:

In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

The given statement is incorrect. The number of protons is never greater than the number of neutrons. The number of neutrons can be equal to or greater than the number of protons because a mass number is equal to double the atomic number or greater than double the atomic number. Also, number of neutrons can be greater than the number of electrons because the number of electrons and protons is always equal in a neutral atom.

#### Question 29:

Calculate the number of neutrons present in the nucleus of an element X which is represented as ${}_{15}{}^{31}\mathrm{X}$.

For the given element X,
Number of protons = Number of electrons = 15
Number of neutrons = Mass number − Number of protons = 31 − 15 = 16
Therefore, the given element X has 16 neutrons.

#### Question 30:

Match the names of the Scientists given in column A with their contributions towards the understanding of the atomic structure as given in column B

 (A) (B) (a) Ernest Rutherford (i) Indivisibility of atoms (b) J.J.Thomson (ii) Stationary orbits (c) Dalton (iii) Concept of nucleus (d) Neils Bohr (iv) Discovery of electrons (e) James Chadwick (v) Atomic number (f) E. Goldstein (vi) Neutron (g) Mosley (vii) Canal rays

(a) - (iii)
(b) - (iv)
(c) - (i)
(d) - (ii)
(e) - (vi)
(f) - (vii)
(g) - (v)

#### Question 31:

The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?

Two or more elements having the same mass number but a different atomic number are known as isobars. Therefore, ${}_{20}{}^{40}\mathrm{Ca}$ and ${}_{18}{}^{40}\mathrm{Ar}$ are pair of isobars.

#### Question 32:

Complete the Table 4.1 on the basis of information available in the symbols given below

(a) ${}_{17}{}^{35}\mathrm{Cl}$            (b) ${}_{6}{}^{12}\mathrm{C}$             (c) ${}_{35}{}^{81}\mathrm{Br}$

 Element np nn

 Element np nn Cl 17 18 C 6 6 Br 35 46

#### Question 33:

Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain.

Helium has only one shell (K shell) and the maximum number of electrons within the K shell can be 2. Thus, its shell is already complete called duplet. It can neither lose electrons nor gain electrons. Hence, its valency is zero. It is called noble gas or inert gas.

#### Question 34:

Fill in the blanks in the following statements
(a) Rutherford’s α-particle scattering experiment led to the discovery of the _________
(b) Isotopes have same _________ but different _________.
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be _________ and _________ respectively.
(d) The electronic configuration of silicon is _________ and that of sulphur is _________.

(a) nucleus
(b) atomic number, mass number
(c) 0 and 1
(d) silicon- 2,8,4 ; sulphur- 2,8,6

#### Question 35:

An element X has a mass number 4 and atomic number 2. Write the valency of this element?

The element X has atomic number 2. Thus, an atom of X contains two electrons in it. These two electrons fill the K shell completely. Thus, the valency of X is zero.

#### Question 36:

Why do Helium, Neon and Argon have a zero valency?

Helium has two electrons which completely fill the K shell. Neon has ten electrons which completely fill K and L shells. Similarly, Argon has 18 electrons which completely K, L and M shell.
The valence shells of these elements are completely filled, thus, they all have stable electronic configuration, and hence, their valency is zero.

#### Question 37:

The ratio of the radii of hydrogen atom and its nucleus is ~ 105. Assuming the atom and the nucleus to be spherical, (i) what will be the ratio of their sizes? (ii) If atom is represented by planet earth ‘Re’ = 6.4 ×106 m, estimate the size of the nucleus.

(i) Volume of the sphere = $\frac{4}{3}{\mathrm{\pi r}}^{3}$
Let R be the radius of the atom and r be that of the nucleus.

$\frac{\mathrm{R}}{\mathrm{r}}={10}^{5}$ (given)

⇒ R = 105

The ratio of the size of the atom to that of the nucleus = $\frac{\frac{4}{3}×{10}^{15}×{\mathrm{\pi r}}^{3}}{\frac{4}{3}{\mathrm{\pi r}}^{3}}={10}^{15}$

(ii) If the atom is represented by the planet earth Re = 6.4 ×106 m

Radius of nucleus = rn$\frac{{\mathrm{R}}_{\mathrm{e}}}{{10}^{5}}$

#### Question 38:

Enlist the conclusions drawn by Rutherford from his α-ray scattering experiment.

Rutherford concluded from α-ray scattering experiment that
1. Most of the space in the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
2. A small fraction of α-particles were deflected indicating that the positive charge of the atom occupies very little space.
3. A very small fraction of α-particles were deflected by 180º, indicating that all the positive charge and mass of the atom were concentrated in a very small volume within the atom.

#### Question 39:

In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?

Thomson's model of the atom proposed the resin pudding structure in which electrons are embedded like resins in a positively charged sphere of pudding or cake which neutralise the positive charge so that, the atom as a whole is electrically neutral.
Rutherford's model could be compared with the solar system. According to his model, the positive charge is concentrated in a very small space in the centre which was called the nucleus. Electrons revolve around the nucleus in well-defined orbits.

#### Question 40:

What were the drawbacks of Rutherford’s model of an atom?

Drawbacks of Rutherford’s model of an atom:
1. A major drawback (or defect) of Rutherford's model of the atom is that it does not explain the stability of the atom.
Any particle in a circular motion would undergo acceleration and would radiate energy. Thus revolving electrons would lose energy and finally fall into the nucleus. Thus, the atom should be unstable and should not exist in a stable form. Rutherford's model, however, could not explain the stability of an atom.
2. The model did not give any information about the arrangement of electrons around the nucleus.

#### Question 41:

What are the postulates of Bohr’s model of an atom?

According to Bohr's model of the structure of the atom
1. In an atom, the electrons revolve rapidly around the nucleus in fixed circular paths called orbits or shells.
2. Only certain special orbits are allowed inside the atom.
3. While revolving in discrete orbits the electrons do not radiate energy. They absorb energy to go to a higher level and emit energy to go to the lower level. These orbits are called energy levels which are represented as K, L, M, N or the numbers, n =1,2,3,4.

#### Question 42:

Show diagramatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Atomic number (Z) of sodium is 11.
Electronic configuration = 2, 8, 1

Sodium ion (Na+) is formed by the loss of one electron from the valence shell.
Electronic configuration = 2, 8 #### Question 43:

In the Gold foil experiment of Geiger and Marsden, that paved the way for Rutherford’s model of an atom, ~ 1.00% of the α-particles were found to deflect at angles > 50º. If one mole of α-particles were bombarded on the gold foil, compute the number of α-particles that would deflect at angles less than 50º.