## Topic outline

• ### Chapter 4 - Structure Of The Atom – 4

• Structure Of The Atom

Q60. 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 > 50°. If one mole of a-particles were bombarded on the gold foil, compute the number of α-particles that would deflect at angles less than 50°.

Ans. % of a-particles deflected more than 50° = 1%

% of a-particles deflected less than 50° = 100 -1 = 99%

Number of a-particles bombarded = 1 mole

1 mole = 6.022 x 1023particles

Number of particles that deflected at an angle less than 50°

= 99/100 x (6.022 x 1023)

= (596.178 x 1023)/100

= 5.96 x 1023

Q61. 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

Ans. Mass number = 27

No. of neutrons = 14

No. of protons = Atomic number

Atomic number = Mass number – No. of neutrons

= 27-14 = 13

No. of electrons = 13

Number of electrons in ion with 3 positive charges = 13 – 3 = 10

Q62. Compare the properties of electrons, protons and neutrons.

Ans.

 Property Electrons Protons Neutrons Relative Charge Negatively charged (-1) Positively charged (+1) No charge (0) Relative Mass Mass is negligible  (1/1840)u Mass is 1u Mass is 1u Absolute Mass 9.8 x 10-28 gram 1.6 x 10-24 gram 1.6 x 10-24 gram Location in the atom Outside the nucleus In the nucleus In the nucleus Discovery J.J Thomson Eugen Goldstein James Chadwick

Q63. What are the limitations of J.J. Thomson’s model of the atom?

Ans. Thomson proposed that:

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.

Q64. Write the distribution of electrons in carbon and sodium atoms.

Ans. Atomic number of carbon = 6

No. of protons = 6

We know that, Number of protons = Number of electrons

Thus the distribution will be

 K L 2 4

Atomic number of sodium = 11

No. of protons = 11

We know that, Number of protons = Number of electrons

Thus the distribution will be

 K L M 2 8 1

Q65. Describe Bohr’s model of the atom.

Ans. Bohr put forward the following postulates about the model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

These orbits or shells are called energy levels. These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

Image from NCERT

Q66. Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Ans. Atomic number of sodium = 11

Mass number of sodium = 23

Number of protons = 11

Number of neutrons = 23 -11 = 12

Number of electrons = 11

Electronic configuration = 2, 8, 1

Sodium ion is formed from sodium atom by loss of 1 electron. Hence, its electronic configuration = 2, 8

Q67. Identify the Mg2+ ion from the figure where, n and p represent the number of neutrons and protons respectively.

Ans. (d)

Explanation

Electronic configuration
atom = 2, 8, 2 and that of Mg2+ ion = 2, 8

Number of protons in Mg atom = 2+ 8 + 2 = 12

Number of neutrons in Mg atom = 24 -12 = 12

Therefore, option (d) is the correct answer.

Q68. What are the postulates of Bohr’s model of an atom?

Ans. Neils Bohr put forward the following postulates about the model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

These orbits or shells are called energy levels. These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

Q69. Enlist the conclusions drawn by Rutherford from his a-ray scattering experiment.

Ans. Rutherford concluded from the α-particle scattering experiment that–

(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

Q70. In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?

Ans. Thomson proposed that:

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Rutherford proposed that:

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

(ii) The electrons revolve around the nucleus in well-defined orbits.

(iii) The size of the nucleus is very small as compared to the size of the atom.