Class 9 Science · Chapter 8

8. Journey Inside the Atom

Complete solutions to Revise, Reflect, Refine. CBSE Class 9 Science (NCERT Exploration).

Revise, Reflect, Refine

1Choose the correct options and explain the reason for the correct and incorrect options in the context of Rutherford’s gold foil experiment.

Correct options: (ii) and (iii)

(i) Incorrect. The experiment did not show neutrons. Neutrons were discovered later by James Chadwick (1932).
(ii) Correct. A few alpha particles bounced back. This could not happen in the plum pudding model. So Rutherford proposed a nucleus at the centre.
(iii) Correct. Large deflections showed that the positive charge and most of the mass are packed in a tiny centre.
(iv) Incorrect. The deflection was caused by the nucleus. It gave no information about the movement of electrons.

2Which of the following statements are correct or incorrect according to Bohr’s atomic model? Give a reason for each.

(i) Incorrect. In a fixed orbit, an electron does not lose energy. So it does not fall into the nucleus.
(ii) Incorrect. Electrons move only in certain fixed orbits, each with a definite energy.
(iii) Correct. This is Bohr’s main idea of stationary orbits (shells).
(iv) Incorrect. Electrons are found only in the energy levels, never in between them.

3X has 18 protons and 19 neutrons. Y has 17 protons and 18 neutrons. Z has 17 protons and 20 neutrons. Explain the relation between (i) Y and Z, (ii) Z and X.

Mass numbers: X = 18 + 19 = 37; Y = 17 + 18 = 35; Z = 17 + 20 = 37

(i) Y and Z have the same atomic number (17) but different mass numbers (35 and 37). They are isotopes.

(ii) Z and X have the same mass number (37) but different atomic numbers (17 and 18). They are isobars.

4What conclusion did Rutherford draw about the position and characteristics of the atom’s positively charged part based on the few alpha particles that bounced back or were deflected at large angles?

The positive charge of an atom is concentrated in a very small region at the centre, called the nucleus.
Almost the entire mass of the atom is in the nucleus.
The nucleus is very small compared to the size of the atom.

5Explain and arrange the statements in the correct chronological order to show how atomic models have evolved.

(iv) Dalton’s model: atoms are indivisible particles.

(ii) Thomson’s model: after the discovery of electrons, the atom was shown as a ‘plum pudding’ with electrons in a sphere of positive charge.

(iii) Rutherford’s model: the gold foil experiment showed a dense central nucleus.

(i) Bohr’s model: electrons move in fixed orbits of definite energy. This explained the stability of the atom.

Correct order: (iv) → (ii) → (iii) → (i)

6Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.

Electrons are negatively charged and the nucleus is positively charged (due to protons). Opposite charges attract each other. This electrostatic force of attraction pulls the electrons towards the nucleus and keeps them in their orbits.

7Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure. Reason (R): The number of electrons is equal to the number of protons in an atom.

Answer: (ii) Both A and R are true, but R is not the correct explanation of A.

R explains why an atom is neutral. It does not explain why the discovery of particles helped us understand the structure.

8For an atom of magnesium with mass number 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and illustrate the arrangement of electrons.

(i) Protons = atomic number = 12
(ii) Neutrons = 24 − 12 = 12
(iii) Electrons = protons = 12
Electronic configuration: 2, 8, 2

Arrangement of electrons in a magnesium atom.

9Find the following information for the elements shown in Fig. 8.17.

Information	(a)	(b)	(c)	(d)
Electron arrangement	2, 1	2, 5	2, 8, 3	2, 7
(i) Name	Lithium	Nitrogen	Aluminium	Fluorine
(ii) Symbol	Li	N	Al	F
(iii) Total electrons	3	7	13	9
(iv) Valence electrons	1	5	3	7
(v) Valency	1	3	3	1
(vi) Protons	3	7	13	9
(vii) Atomic number	3	7	13	9

10Both Rutherford’s and Bohr’s models have electrons orbiting the nucleus. Why did Rutherford’s model fail to explain atomic stability, while Bohr’s model succeeded?

Rutherford’s model: a revolving electron should lose energy continuously. It would move in a spiral path and fall into the nucleus. Then atoms would be unstable. But atoms are stable.

Bohr’s model: electrons move only in certain fixed orbits (stationary states). While moving in a fixed orbit, an electron does not lose energy. So it does not fall into the nucleus, and the atom stays stable.

11An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?

Protons = electrons = 31
Neutrons = 70 − 31 = 39

12An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.

(i) Neutrons = 197 − 79 = 118
(ii) Electrons = protons = 79

13Complete Table 8.5.

Atomic number	Mass number	Neutrons	Protons	Electrons	Name of the element
5	11	6	5	5	Boron
7	14	7	7	7	Nitrogen
12	24	12	12	12	Magnesium
15	31	16	15	15	Phosphorus
1	1	0	1	1	Hydrogen

14Element X has a mass number of 35 and contains 18 neutrons. Answer the following.

(i) Protons = 35 − 18 = 17; Electrons = 17
(ii) Atomic number = 17
(iii) Element X = Chlorine (Cl)
(iv) Electronic configuration = 2, 8, 7
(v) Valence electrons = 7
(vi) New mass number = 35 + 2 = 37
(vii) Same atomic number, different mass number ⇒ they are isotopes

15An atom has 12 protons and 12 neutrons. All its electrons are replaced by hypothetical particles with the same charge as electrons but 500 times heavier. What effect will this have on its (i) atomic number, (ii) atomic mass, (iii) mass number, (iv) overall charge?

(i) Atomic number: no change. It stays 12, as the number of protons is the same.
(ii) Atomic mass: increases. Electrons have very little mass, but 12 particles each 500 times heavier add noticeable mass (about 3 u). So the atomic mass becomes about 27 u instead of about 24 u.
(iii) Mass number: no change. It stays 24, as it counts only protons and neutrons.
(iv) Overall charge: no change. The atom stays neutral, because the new particles have the same charge as electrons.

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