Marking Scheme / Answer Key
Topic: Structure of the Atom | Class: IX
SECTION - A (1 Mark Each)
1. (b) J.J. Thomson
2. (d) 2, 8, 1
(Na has 11 electrons. K=2, L=8, M=1)
(Na has 11 electrons. K=2, L=8, M=1)
3. (c) Different number of neutrons
4. (a) Atomic Nucleus
5. (b) 8
(Formula 2n². For L shell, n=2, so 2(4) = 8)
(Formula 2n². For L shell, n=2, so 2(4) = 8)
SECTION - B (Short Answer Type)
6. Valency:
Definition: The combining capacity of an atom.
Magnesium (Z=12, Config: 2,8,2): Loses 2 electrons. Valency = 2.
Chlorine (Z=17, Config: 2,8,7): Gains 1 electron. Valency = 1.
Magnesium (Z=12, Config: 2,8,2): Loses 2 electrons. Valency = 2.
Chlorine (Z=17, Config: 2,8,7): Gains 1 electron. Valency = 1.
7. Bohr's Model Sketch:
Diagram should show:
1. Central positively charged Nucleus.
2. Circular orbits labeled K (n=1), L (n=2), M (n=3) moving outward from the center.
1. Central positively charged Nucleus.
2. Circular orbits labeled K (n=1), L (n=2), M (n=3) moving outward from the center.
8. Rutherford's Observations:
1. Most α-particles passed straight through the gold foil.
2. A tiny fraction (1 in 12,000) bounced back (rebounded 180°).
3. Some particles were deflected by small angles (indicating positive charge center).
2. A tiny fraction (1 in 12,000) bounced back (rebounded 180°).
3. Some particles were deflected by small angles (indicating positive charge center).
9. Bohr's Postulates:
1. Electrons revolve only in certain special orbits known as discrete orbits.
2. While revolving in these discrete orbits, electrons do not radiate energy (ensuring stability).
2. While revolving in these discrete orbits, electrons do not radiate energy (ensuring stability).
10. Isobars & Neutrality:
(a) Isobars: Atoms of different elements with different Atomic Numbers (Z) but the same Mass Number (A). Example: Calcium (40) and Argon (40).
(b) Neutrality: The negative charge of electrons and positive charge of protons are equal in magnitude and balance each other out.
(b) Neutrality: The negative charge of electrons and positive charge of protons are equal in magnitude and balance each other out.
SECTION - C (Long Answer Type)
11. Comparison of Particles:
| Particle | Charge | Mass | Location |
|---|---|---|---|
| Electron | Negative (-1) | Negligible (1/2000 of p+) | Orbiting nucleus |
| Proton | Positive (+1) | 1 Unit | Inside Nucleus |
| Neutron | Neutral (0) | 1 Unit | Inside Nucleus |
12. Isotopes of Hydrogen:
Explanation: Atoms of the same element with same Z but different A (different neutrons).
The Three Isotopes:
1. Protium (1 p, 0 n)
2. Deuterium (1 p, 1 n)
3. Tritium (1 p, 2 n)
Medical Application (Any one):
- Cobalt isotope for cancer treatment.
- Iodine isotope for treating goitre.
The Three Isotopes:
1. Protium (1 p, 0 n)
2. Deuterium (1 p, 1 n)
3. Tritium (1 p, 2 n)
Medical Application (Any one):
- Cobalt isotope for cancer treatment.
- Iodine isotope for treating goitre.
