Structure of the Atom
Electrons, protons, neutrons; Thomson, Rutherford, and Bohr models.
Chapter Overview
From Thomson's plum pudding to Rutherford's nuclear model and Bohr's orbits, this chapter traces discovery of subatomic particles, atomic number, mass number, isotopes, and electronic configuration.
Subatomic Particles
- Electron (e⁻): −1 charge, negligible mass
- Proton (p⁺): +1 charge, mass ≈ 1 u
- Neutron (n): no charge, mass ≈ 1 u
Atomic Notation
Mass number
A = protons + neutrons
Atomic number
Z = number of protons = electrons (neutral atom)
Valency
Electrons in outermost shell determine combining capacity
Bohr shells
2n² electrons max in shell n (K=2, L=8, M=18...)
Isotopes & Isobars
- Isotopes: same Z, different A (¹H, ²H, ³H)
- Isobars: same A, different Z (⁴⁰Ar, ⁴⁰Ca)
- Isotopes of Cl used in average atomic mass
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Find neutrons
An atom has Z=11, A=23. Find protons, electrons, neutrons.
- 1Protons = Z = 11
- 2Electrons = 11 (neutral)
- 3Neutrons = A − Z = 12
Answer
11, 11, 12
Electronic configuration
Configuration of sodium (Z=11).
- 1K shell: 2
- 2L shell: 8
- 3M shell: 1
- 4Valency = 1
Answer
2, 8, 1
Key Points to Remember
- ✓ Rutherford: nucleus is tiny, dense, positively charged
- ✓ Most of atom is empty space
- ✓ Bohr model explains line spectra of hydrogen
Exam Tips
- • Draw Bohr model for first 20 elements
- • Distinguish isotopes, isobars, isotones
- • Know discovery experiments (cathode rays, gold foil)
Practice MCQs — Structure of the Atom
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Rutherford's experiment used: