Journey Inside the Atom – Class 9 Science notes, important questions and MCQs

Chapter overview

Atoms are made of protons, neutrons and electrons. Models by Thomson, Rutherford and Bohr explain how these particles are arranged.

Sub-atomic particles

• Electron (e⁻): negative, discovered by J. J. Thomson; very light.
• Proton (p⁺): positive, in the nucleus (Goldstein's canal rays).
• Neutron (n): no charge, in the nucleus, discovered by Chadwick.

Atomic models

• Thomson: positive sphere with electrons embedded (plum pudding).
• Rutherford (gold foil): most of the atom is empty; a tiny dense positive nucleus at the centre.
• Bohr: electrons move in fixed shells (K, L, M, N) without losing energy.

Electron arrangement

• Maximum electrons in a shell = 2n²: K = 2, L = 8, M = 18.
• Valence electrons (outermost) decide chemical properties and valency.
• Atomic number Z = protons; mass number A = protons + neutrons.
• Isotopes: same Z, different A (C-12, C-14). Isobars: same A, different Z (Ar-40, Ca-40).

Key terms

Nucleus

the dense central part of an atom with protons and neutrons

Valency

combining capacity of an atom

Isotopes

atoms of the same element with different mass numbers

Formulas

• Max electrons in shell n = 2n²
• Neutrons = A − Z

Important questions with answers

Quick revision

• Rutherford discovered the nucleus.
• Neutron has no charge.
• Isotopes have the same chemical properties.

Exam tips

• Write electronic configuration as 2, 8, 1 (sodium) — fill shells in order.
• Valency = valence electrons (if ≤ 4) or 8 − valence electrons.

Practice MCQs

1. The particle with no charge is the

   1. neutron
   2. proton
   3. electron
   4. ion
   Show answer

   Answer: (a) neutron. Neutron.

2. Who discovered the electron?

   1. Rutherford
   2. Chadwick
   3. J. J. Thomson
   4. Bohr
   Show answer

   Answer: (c) J. J. Thomson. Cathode ray experiments.

3. The K shell can hold at most

   1. 2 electrons
   2. 8 electrons
   3. 18 electrons
   4. 32 electrons
   Show answer

   Answer: (a) 2 electrons. 2n² with n = 1.

4. Electronic configuration of sodium (Z = 11) is

   1. 2, 9
   2. 2, 8, 1
   3. 8, 2, 1
   4. 2, 7, 2
   Show answer

   Answer: (b) 2, 8, 1. 2 + 8 + 1.

5. An atom has 17 protons and 18 neutrons. Its mass number is

   1. 35
   2. 18
   3. 17
   4. 1
   Show answer

   Answer: (a) 35. 17 + 18.

6. Rutherford's gold foil experiment showed that

   1. most of the atom is empty space
   2. electrons are in shells
   3. neutrons exist
   4. atoms are indivisible
   Show answer

   Answer: (a) most of the atom is empty space. Most alpha particles passed straight through.

7. The valency of chlorine (2, 8, 7) is

   1. 7
   2. 1
   3. 2
   4. 8
   Show answer

   Answer: (b) 1. Needs 1 electron: 8 − 7 = 1.

8. ⁴⁰Ar and ⁴⁰Ca are

   1. isotopes
   2. isomers
   3. isobars
   4. allotropes
   Show answer

   Answer: (c) isobars. Same mass number, different atomic numbers.