Atomic Theory and Models
~9 min read
- Three particles: Proton (+, ~1 u), Neutron (0, ~1 u), Electron (-, 1/1836 u).
- Atomic number Z: Number of protons. Defines element. Mass number A = p + n.
- Models: Dalton (1803) → Thomson plum-pudding (1897) → Rutherford nuclear (1911) → Bohr orbits (1913) → Quantum mechanical (1920s).
Atomic theory is the spine of chemistry. CDS/OTA tests sub-atomic particles, discovery experiments, and the evolution of atomic models from Dalton to quantum mechanics.
Sub-Atomic Particles
| Particle | Symbol | Charge | Mass | Discovered by |
|---|---|---|---|---|
| Electron | e⁻ | -1 | 1/1836 u | J.J. Thomson, 1897 (cathode ray) |
| Proton | p | +1 | 1 u | Goldstein 1886, named by Rutherford 1920 |
| Neutron | n | 0 | 1 u | James Chadwick, 1932 |
Evolution of Atomic Models
| Year | Scientist | Idea |
|---|---|---|
| 1803 | John Dalton | Atoms are tiny, indivisible spheres. Each element has its own kind. |
| 1897 | J.J. Thomson | Plum-pudding — sphere of positive charge with electrons embedded. |
| 1911 | Rutherford | Gold-foil experiment. Atom mostly empty with small dense nucleus. |
| 1913 | Niels Bohr | Electrons in fixed circular orbits (energy levels). Jumps emit/absorb photons. |
| 1920s | Schrödinger, Heisenberg | Quantum mechanical — electrons as probability clouds (orbitals). |
Electron Shells (Bohr-Bury)
- Shells labelled K, L, M, N (n = 1, 2, 3, 4 …).
- Maximum capacity = 2n². K = 2, L = 8, M = 18, N = 32.
- Outermost shell is the valence shell.
- Octet rule: atoms tend to have 8 electrons in valence shell (or 2 for first shell — duet).
- Examples — H (1), C (2,4), Na (2,8,1), Cl (2,8,7), Ar (2,8,8).
CDS/OTA PYQ Examples
Q: Neutron was discovered by:
(a) Rutherford (b) Thomson (c) Chadwick (d) Bohr
Answer: (c) James Chadwick (1932).
Q: Atomic number is the number of:
(a) Neutrons (b) Protons (c) Electrons + neutrons (d) Nucleons
Answer: (b) Protons.
Q: In Rutherford's gold-foil experiment, most alpha particles:
(a) Were absorbed (b) Passed straight through (c) Bounced back (d) Were split
Answer: (b) Passed straight through — proving atom is mostly empty space.
Q: Maximum number of electrons in M shell is:
(a) 2 (b) 8 (c) 18 (d) 32
Answer: (c) 18 (= 2 × 3²).
Drill Atomic Theory and Models for CDS/OTA
CDS/OTA-pattern items on Atomic Theory and Models with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is the atom electrically neutral?
Equal number of protons and electrons cancel each other's charges.
Why did Rutherford's model fail?
Classical physics predicted that orbiting electrons would radiate energy and spiral into the nucleus. Bohr fixed this by postulating quantised orbits.
Why is the Bohr model still taught despite quantum mechanics?
It explains the hydrogen spectrum well, introduces quantisation simply, and is a good first approximation for one-electron systems.