📘Class 9 Science – Chapter: Structure of the Atom Notes
✨ Introduction
All matter is made up of atoms.
Atoms are composed of subatomic particles: electrons, protons, and neutrons.
Understanding atomic structure explains how atoms combine and react.
🔹 Charged Particles in Matter
Electrons were discovered by J.J. Thomson through the cathode ray experiment.
Protons were discovered by E. Goldstein using canal rays.
Neutrons were discovered by James Chadwick in 1932.
| Particle | Symbol | Charge | Mass (approx.) | Location | Discovered by |
|---|---|---|---|---|---|
| Electron | e⁻ | -1 | 9.1 × 10^-31 kg | Outside nucleus | J.J. Thomson |
| Proton | p⁺ | +1 | 1.67 × 10^-27 kg | In nucleus | E. Goldstein |
| Neutron | n⁰ | 0 (neutral) | 1.67 × 10^-27 kg | In nucleus | James Chadwick |
🌐 Models of the Atom
1. Thomson’s Model (Plum Pudding Model)
Atom is a positively charged sphere with negatively charged electrons embedded in it like plums in pudding.
Could not explain atomic stability.
2. Rutherford’s Nuclear Model
Based on gold foil experiment:
α-particles were directed at a thin sheet of gold.
Most passed through; few deflected.
Concluded that atom is mostly empty space with dense nucleus at center.
Key Features:
Nucleus: Small, dense, positively charged.
Electrons revolve in orbits.
Drawbacks:
Could not explain why electrons do not spiral into the nucleus.
3. Bohr’s Model
Electrons revolve in fixed energy orbits (called shells or energy levels).
Each orbit is associated with definite energy.
Electrons emit/absorb energy only when jumping between orbits.
📈 Diagrams
Bohr’s Model of Hydrogen Atom:
Nucleus
|
-----------
| |
e⁻ (K-shell)
Bohr’s Model of Sodium Atom (Na):
Atomic number = 11
Electron configuration = 2, 8, 1
Nucleus (11p+, 12n0)
/ | \
K-shell L-shell M-shell
(2e⁻) (8e⁻) (1e⁻)
🔹 Structure of an Atom
| Particle | Location | Relative Mass | Relative Charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Outside nucleus | 1/1836 | -1 |
Nucleus: Positively charged, dense center of atom containing protons and neutrons.
Shells: Electrons revolve around the nucleus in fixed orbits named K, L, M, N…
📊 Atomic Number (Z) and Mass Number (A)
Atomic Number (Z): Number of protons in an atom. Also equals number of electrons in a neutral atom.
Mass Number (A): Total number of protons + neutrons in the nucleus.
Formula:
Number of neutrons=A−Z\text{Number of neutrons} = A – Z
Example:
Carbon:
Z = 6, A = 12
Protons = 6, Neutrons = 6, Electrons = 6
🎓 Isotopes and Isobars
Isotopes:
Atoms of the same element with same atomic number but different mass numbers.
They have same chemical properties but different physical properties.
| Isotope | Atomic Number | Mass Number | Neutrons | Use |
|---|---|---|---|---|
| Hydrogen-1 | 1 | 1 | 0 | Most common |
| Hydrogen-2 | 1 | 2 | 1 | Used in nuclear fusion |
| Hydrogen-3 | 1 | 3 | 2 | Used in radioactive tracing |
Isobars:
Atoms of different elements with same mass number but different atomic numbers.
| Element | Atomic Number | Mass Number | Neutrons |
|---|---|---|---|
| Calcium (Ca) | 20 | 40 | 20 |
| Argon (Ar) | 18 | 40 | 22 |
🔹 Electron Distribution (Bohr-Bury Scheme)
Maximum number of electrons in a shell: 2n22n^2
Where n = shell number (1 for K, 2 for L, etc.)
| Shell | Symbol | Max Electrons (2n^2) |
|---|---|---|
| 1 | K | 2 |
| 2 | L | 8 |
| 3 | M | 18 |
| 4 | N | 32 |
Rules:
Filling starts from the innermost shell.
Maximum 8 electrons in outermost shell.
Outer shell is not filled until inner shells are filled.
🔹 Valency
The combining capacity of an atom.
Determined by the number of electrons in the outermost shell.
Rules:
If outer shell < 4 electrons, valency = number of electrons.
If outer shell > 4, valency = 8 – number of electrons.
Examples:
| Element | Atomic No. | Electron Configuration | Outermost e⁻ | Valency |
|---|---|---|---|---|
| H | 1 | 1 | 1 | 1 |
| O | 8 | 2, 6 | 6 | 2 |
| N | 7 | 2, 5 | 5 | 3 |
| Na | 11 | 2, 8, 1 | 1 | 1 |
| Cl | 17 | 2, 8, 7 | 7 | 1 |