📘 Chapter 4: Structure of the Atom – (CBSE Class 9 Science)
🔹 I. Introduction
Atoms were once thought to be indivisible, but discovery of subatomic particles — electrons, protons, and neutrons — revealed that atoms have internal structure.
🔹 II. Subatomic Particles
| Particle | Symbol | Charge | Mass (approx) | Discovered By |
|---|---|---|---|---|
| Electron | e⁻ | –1 | 9.1 × 10⁻³¹ kg | J.J. Thomson |
| Proton | p⁺ | +1 | 1.67 × 10⁻²⁷ kg | E. Goldstein |
| Neutron | n⁰ | 0 | 1.67 × 10⁻²⁷ kg | James Chadwick |
🔹 III. Atomic Models
🔸 1. Thomson’s Model (Plum Pudding Model)
- Atom is a sphere of positive charge with negatively charged electrons embedded.
- Like a pudding with raisins.
- Limitation: Couldn’t explain nucleus or scattering experiments.
🔸 2. Rutherford’s Scattering Experiment
- Alpha particle experiment on thin gold foil.
- Observations:
- Most alpha particles passed straight.
- Some deflected at small angles.
- Few bounced back.
- Conclusions:
- Atom is mostly empty.
- Nucleus is small, dense, positively charged.
- Electrons revolve around nucleus.
Limitation: Couldn’t explain stability of atom (why electrons don’t fall into nucleus).
🔹 IV. Bohr’s Model of Atom
| Key Postulates: |
|---|
- Electrons revolve in fixed orbits (shells).
- Each orbit has fixed energy (energy levels).
- No radiation is emitted unless electron jumps orbits.
Energy Levels/Shells: K (n=1), L (n=2), M (n=3), N (n=4)…
🔹 V. Neutron Discovery
- Discovered by James Chadwick (1932).
- Neutrons add mass to the atom but no charge.
🔹 VI. Atomic Number (Z)
- Z = Number of protons
- In neutral atom: Number of electrons = Number of protons
Example:
- Hydrogen (Z=1) → 1 proton, 1 electron
- Carbon (Z=6) → 6 protons, 6 electrons
🔹 VII. Mass Number (A)
- A = Number of protons + Number of neutrons
No. of neutrons=A−Z\text{No. of neutrons} = A – ZNo. of neutrons=A−Z
Example:
- Oxygen-16 (Z=8, A=16) → 8 neutrons
🔹 VIII. Isotopes and Isobars
🔸 Isotopes:
- Atoms of same element with same atomic number but different mass number.
- Example: Hydrogen-1 (Protium), Hydrogen-2 (Deuterium), Hydrogen-3 (Tritium)
Uses:
- Isotope of Iodine: Treat thyroid.
- Isotope of Cobalt: Cancer therapy.
🔸 Isobars:
- Atoms of different elements with same mass number but different atomic numbers.
- Example: Calcium-40 (Z=20) and Argon-40 (Z=18)
🔹 IX. Distribution of Electrons (Bohr-Bury Rule)
- Max electrons in a shell: 2n² (n = shell number)
| Shell | n | Max electrons |
|---|---|---|
| K | 1 | 2 |
| L | 2 | 8 |
| M | 3 | 18 |
| N | 4 | 32 |
- Outer shell can’t have more than 8 electrons.
- Next shell starts filling only after inner shell is full.
🔹 X. Electronic Configuration
Examples:
| Element | Z | Configuration |
|---|---|---|
| H | 1 | 1 |
| He | 2 | 2 |
| Li | 3 | 2,1 |
| Be | 4 | 2,2 |
| B | 5 | 2,3 |
| C | 6 | 2,4 |
| O | 8 | 2,6 |
| Na | 11 | 2,8,1 |
| Cl | 17 | 2,8,7 |
🔹 XI. Valency
- Valency = number of electrons lost, gained, or shared to complete octet.
- Valency = 8 – outermost electrons (if > 4)
- Valency = outermost electrons (if ≤ 4)
| Element | Configuration | Valency |
|---|---|---|
| H | 1 | 1 |
| C | 2,4 | 4 |
| O | 2,6 | 2 |
| Na | 2,8,1 | 1 |
| Cl | 2,8,7 | 1 |
🔹 XII. Mind Map – Structure of the Atom
yamlCopyEdit STRUCTURE OF THE ATOM
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Subatomic Models Atomic No Mass No
Particles (JJ, R, B) (Z) (A)
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Isotopes & Isobars Electronic Configuration
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Valency
🔹 XIII. NCERT Sample Questions
Q1. How many electrons, protons, and neutrons are there in an atom of:
(i) He (Atomic No 2, Mass No 4)?
- Electrons = 2, Protons = 2, Neutrons = 4 – 2 = 2
Q2. Give the electronic configuration of Na (Z=11):
- 2, 8, 1
Q3. Define Isotopes. Give two examples.
- Atoms with same atomic number but different mass numbers.
- Ex: H-1, H-2, H-3; C-12, C-14