Topics Covered
10 key topics in this chapter
Study Resources
Key Concepts
01. Discovery of Sub-atomic Particles
J.J. Thomson (1897) discovered electrons using cathode ray tube — negatively charged. E. Goldstein (1886) discovered protons (canal rays — positively charged). J. Chadwick (1932) discovered neutrons — neutral.
02. Thomson's Plum-Pudding Model
Thomson proposed an atom as a uniform sphere of positive charge with electrons (plums) embedded in it. This model could not explain Rutherford's scattering results.
03. Rutherford's Gold Foil Experiment
Most α-particles passed straight through, some deflected, very few bounced back. Conclusion: atom is mostly empty space with a tiny, dense, positive nucleus. Limitation: could not explain stable orbits.
04. Bohr's Atomic Model
Electrons revolve in fixed circular orbits (shells) without radiating energy. Each shell has a fixed energy. Electrons jump between shells by absorbing/emitting quanta of energy. Shells are labelled K, L, M, N (n=1,2,3,4).
05. Electronic Configuration
Maximum electrons in shell n = 2n². So K(n=1) → 2, L(n=2) → 8, M(n=3) → 18. Outermost shell can hold max 8 electrons. Configuration is written as 2, 8, 3 (for Al, Z=13).
06. Atomic Number, Mass Number, Isotopes & Isobars
Atomic number Z = number of protons. Mass number A = protons + neutrons (Z + N). Isotopes: same Z, different A (¹H, ²H, ³H). Isobars: same A, different Z (⁴⁰Ca and ⁴⁰Ar).
Formulas at a Glance
| # | Name | Expression | Notes |
|---|---|---|---|
| 01 | Mass Number | A = Z + N |
Protons (Z) + Neutrons (N) |
| 02 | Number of Neutrons | N = A − Z |
From mass and atomic numbers |
| 03 | Shell Capacity | 2n² |
Max electrons in nth shell |
| 04 | Valency (metal/nonmetal) | V = outermost electrons (or 8 − outermost) |
For electrons < 4, V = e⁻; for > 4, V = 8 − e⁻ |
Important Notes
Exam Tips & Common Mistakes
For "draw the electronic configuration of element X" questions: fill shells as 2, 8, 8… until electrons run out.
Distinguish Isotopes vs Isobars: same Atomic number = Isotopes; same Mass number = Isobars (one-mark distinction).
Rutherford's experiment — know the three observations and the three corresponding conclusions. This is a 5-mark question pattern.
Bohr's model limitation: applies accurately only to hydrogen atom; fails for multi-electron atoms.
Valency of elements with 5, 6, 7 outer electrons = 3, 2, 1 respectively (8 - outer electrons).