Class 9 - Chapter 8: Journey Inside the Atom

Exercise Solution

1. Choose the correct options and explain the reasons

(i) The experiment clearly showed the existence of neutrons in the nucleus.

Incorrect

Rutherford's gold foil experiment did not discover neutrons. Neutrons were discovered later by James Chadwick in 1932.

(ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.

Correct

The experiment showed that most alpha particles passed straight through while a few were deflected, proving the existence of a small dense nucleus.

(iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre.

Correct

Large deflections could occur only if a heavy, positively charged nucleus existed at the centre.

(iv) The way alpha particles were deflected showed that electrons move around the nucleus.

Incorrect

The experiment gave information about the nucleus but did not provide direct evidence about electron motion.

2. Bohr's Atomic Model

(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.

Incorrect

According to Bohr, electrons in permitted orbits do not lose energy.

(ii) Electrons can exist anywhere around the nucleus with no fixed energy.

Incorrect

Electrons can occupy only certain fixed energy levels.

(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy.

Correct

This is the main postulate of Bohr's atomic model.

(iv) Electrons can be found between energy levels as they move around the nucleus.

Incorrect

Electrons cannot exist between two energy levels.

3. Relationship between X, Y and Z

(i) Y and Z

Y and Z have the same number of protons (17) but different numbers of neutrons.

Therefore, Y and Z are isotopes.

(ii) Z and X

Z and X have different numbers of protons and different mass numbers.

They are different elements and have no special isotope relationship.

4. Rutherford's conclusion about the nucleus

From the large-angle deflection and bouncing back of a few alpha particles, Rutherford concluded:

• The positive charge of the atom is concentrated in a very small region.
• This region is called the nucleus.
• The nucleus contains most of the mass of the atom.
• Most of the atom is empty space.

5. Correct chronological order of atomic models

(iv) Dalton's model

(ii) Thomson's model

(iii) Rutherford's model

(i) Bohr's model

Correct Order:

Dalton → Thomson → Rutherford → Bohr

6. Why do electrons not fly away from the atom?

Electrons are negatively charged while the nucleus is positively charged.

The electrostatic force of attraction between the nucleus and electrons keeps the electrons bound to the atom and prevents them from flying away.

7. Assertion–Reason

Assertion (A): The discovery of subatomic particles helped in understanding atomic structure.

Reason (R): The number of electrons is equal to the number of protons in an atom.

Both statements are true, but the reason does not explain the assertion.

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

8. Magnesium Atom

Mass number = 24

Atomic number = 12

(i) Number of protons

= Atomic number

= 12

(ii) Number of neutrons

= Mass number − Atomic number

= 24 − 12

= 12

(iii) Number of electrons

= Number of protons

= 12

Electronic Configuration

2, 8, 2

9. Information from Fig. 8.17

(a)

Electrons = 2

Element = Helium (He)

Valence electrons = 2

Valency = 0

Protons = 2

Atomic number = 2

(b)

Electronic configuration = 2, 6

Element = Oxygen (O)

Total electrons = 8

Valence electrons = 6

Valency = 2

Protons = 8

Atomic number = 8

(c)

Electronic configuration = 2, 8, 2

Element = Magnesium (Mg)

Total electrons = 12

Valence electrons = 2

Valency = 2

Protons = 12

Atomic number = 12

(d)

Electronic configuration = 2, 7

Element = Fluorine (F)

Total electrons = 9

Valence electrons = 7

Valency = 1

Protons = 9

Atomic number = 9

10. Why Rutherford's model failed but Bohr's succeeded?

According to Rutherford's model, revolving electrons should continuously lose energy and eventually fall into the nucleus, making the atom unstable.

Bohr solved this problem by proposing fixed energy levels in which electrons can revolve without losing energy.

11. Number of neutrons in ⁷⁰X

Mass number = 70

Number of electrons = 31

Therefore atomic number = 31

Number of neutrons = 70 − 31

= 39

Answer: 39 neutrons

12. Atom with 79 protons and mass number 197

(i) Number of neutrons

= 197 − 79

= 118

(ii) Number of electrons

= Number of protons

= 79

13. Complete Table 8.5

14. Element X

Mass number = 35

Neutrons = 18

(i) Number of electrons and protons

Protons = 35 − 18 = 17

Electrons = 17

(ii) Atomic number

17

(iii) Identify the element

Chlorine (Cl)

(iv) Electronic configuration

2, 8, 7

(v) Valence electrons

7

(vi) New mass number after adding 2 neutrons

35 + 2 = 37

(vii) Relation between X and new atom

They are isotopes of chlorine.

15. Replacing electrons with particles 500 times heavier

Protons = 12

Neutrons = 12

(i) Atomic number

Unchanged = 12

(ii) Atomic mass

Increases because the new particles are much heavier than electrons.

(iii) Mass number

Unchanged = 24

(Mass number depends only on protons and neutrons.)

(iv) Overall charge

Remains neutral because the new particles have the same negative charge as electrons.