Chemistry · Class 9-10 · CBSE
Structure of the Atom: Thomson, Rutherford and Bohr Models
Understand subatomic particles (electrons, protons, neutrons), Thomson’s plum pudding model, Rutherford’s alpha-particle scattering experiment and nuclear model, Bohr’s atomic postulates, electronic configuration shells (2n² rule), valency, isotopes and isobars.
1Subatomic Particles and Thomson’s Model
Dalton’s theory regarded the atom as indivisible, but late 19th-century discoveries revealed three fundamental subatomic particles:Electrons(J.J. Thomson, cathode rays),Protons(E. Goldstein, canal rays), andNeutrons(James Chadwick, 1932).
The Three Subatomic Particles
| Particle | Discoverer | Relative Charge | Absolute Mass (kg) | Location in Atom |
|---|---|---|---|---|
| Electron (e⁻) | J.J. Thomson (1897) | -1 | 9.109 × 10⁻³¹ kg (~1/1840 u) | Extra-nuclear orbits / shells |
| Proton (p⁺) | E. Goldstein (1886) | +1 | 1.672 × 10⁻²⁷ kg (1.007 u) | Inside the central nucleus |
| Neutron (n⁰) | James Chadwick (1932) | 0 (Neutral) | 1.675 × 10⁻²⁷ kg (1.008 u) | Inside the central nucleus |
Electron (e⁻)
- Discoverer
- J.J. Thomson (1897)
- Relative Charge
- -1
- Absolute Mass (kg)
- 9.109 × 10⁻³¹ kg (~1/1840 u)
- Location in Atom
- Extra-nuclear orbits / shells
Proton (p⁺)
- Discoverer
- E. Goldstein (1886)
- Relative Charge
- +1
- Absolute Mass (kg)
- 1.672 × 10⁻²⁷ kg (1.007 u)
- Location in Atom
- Inside the central nucleus
Neutron (n⁰)
- Discoverer
- James Chadwick (1932)
- Relative Charge
- 0 (Neutral)
- Absolute Mass (kg)
- 1.675 × 10⁻²⁷ kg (1.008 u)
- Location in Atom
- Inside the central nucleus
2Rutherford’s Scattering Experiment vs Bohr’s Model
Rutherford’s Alpha Particle Gold Foil Experiment (1911)
- 1
1. Observation 1: Most alpha particles passed straight through the gold foil undeflected ⇒ Most of the space inside the atom is empty.
- 2
2. Observation 2: A few alpha particles deflected by small and large angles ⇒ Positive charge is concentrated in a tiny core, not spread throughout.
- 3
3. Observation 3: 1 in 12,000 rebounded straight back (180° deflection) ⇒ Entire mass and positive charge is packed into an extremely dense central Nucleus.
- 4
4. Major Drawback: According to Maxwell’s electromagnetic theory, revolving electrons must accelerate, radiate energy, spiral inwards, and collapse into nucleus in 10⁻⁸ s (atom would be unstable).
Bohr’s Postulates Overcoming Rutherford’s Drawback
| Shell Letter | Principal Quantum Number (n) | Max Electron Capacity (2n²) | Radiation Energy Behavior |
|---|---|---|---|
| K Shell | 2(1)² = 2 electrons | Non-radiating discrete orbit (Stationary state) | |
| L Shell | 2(2)² = 8 electrons | Electron does not radiate energy while in orbit | |
| M Shell | 2(3)² = 18 electrons | Energy radiated only when dropping to lower shell | |
| N Shell | 2(4)² = 32 electrons | Energy absorbed when jumping to higher shell |
K Shell
- Principal Quantum Number (n)
- Max Electron Capacity (2n²)
- 2(1)² = 2 electrons
- Radiation Energy Behavior
- Non-radiating discrete orbit (Stationary state)
L Shell
- Principal Quantum Number (n)
- Max Electron Capacity (2n²)
- 2(2)² = 8 electrons
- Radiation Energy Behavior
- Electron does not radiate energy while in orbit
M Shell
- Principal Quantum Number (n)
- Max Electron Capacity (2n²)
- 2(3)² = 18 electrons
- Radiation Energy Behavior
- Energy radiated only when dropping to lower shell
N Shell
- Principal Quantum Number (n)
- Max Electron Capacity (2n²)
- 2(4)² = 32 electrons
- Radiation Energy Behavior
- Energy absorbed when jumping to higher shell
3Valency, Atomic Number, Isotopes and Isobars
Isotopes vs Isobars: Comprehensive Comparison
| Feature | Isotopes | Isobars |
|---|---|---|
| Definition | Atoms of same element having SAME atomic number (Z), but DIFFERENT mass number (A) | Atoms of different elements having DIFFERENT atomic number (Z), but SAME mass number (A) |
| Protons & Electrons | Identical number of protons and electrons | Different number of protons and electrons |
| Neutrons | Different number of neutrons (A - Z) | Different number of neutrons |
| Chemical Properties | Identical chemical properties (same valence electrons) | Completely different chemical properties (different elements) |
| Classic Examples | Protium (¹H), Deuterium (²H), Tritium (³H); Carbon-12 and Carbon-14 | Argon (₁₈Ar⁴⁰) and Calcium (₂₀Ca⁴⁰) |
Definition
- Isotopes
- Atoms of same element having SAME atomic number (Z), but DIFFERENT mass number (A)
- Isobars
- Atoms of different elements having DIFFERENT atomic number (Z), but SAME mass number (A)
Protons & Electrons
- Isotopes
- Identical number of protons and electrons
- Isobars
- Different number of protons and electrons
Neutrons
- Isotopes
- Different number of neutrons (A - Z)
- Isobars
- Different number of neutrons
Chemical Properties
- Isotopes
- Identical chemical properties (same valence electrons)
- Isobars
- Completely different chemical properties (different elements)
Classic Examples
- Isotopes
- Protium (¹H), Deuterium (²H), Tritium (³H); Carbon-12 and Carbon-14
- Isobars
- Argon (₁₈Ar⁴⁰) and Calcium (₂₀Ca⁴⁰)
4Exam Corner: Tips, Mistakes & Board Questions
High-Yield Applications of Isotopes
- Uranium-235 isotope is used as fuel in nuclear reactors.
- Cobalt-60 isotope is used in radiotherapy for cancer treatment.
- Iodine-131 isotope is used in the treatment and imaging of goitre (thyroid disorder).
Exam question · CBSE Class 9/10 Board
31. How many electrons, protons, and neutrons does it have, and what is its valency?
- A.15 e⁻, 15 p⁺, 16 n⁰; Valency = 3
- B.15 e⁻, 15 p⁺, 31 n⁰; Valency = 5
- C.16 e⁻, 15 p⁺, 15 n⁰; Valency = 1
- D.15 e⁻, 16 p⁺, 15 n⁰; Valency = 3
- Number of electrons in neutral atom = Z = 15.
- Electronic configuration: (2, 8, 5). Valence electrons = 5.
- Correct answer is option A.
Answer: A. 15 e⁻, 15 p⁺, 16 n⁰; Valency = 3
Remember
- Atom consists ofProtons and Neutrons in nucleuswithElectrons in quantized orbits.
- Bohr proved electrons revolve instationary non-radiating discrete orbits.
- Maximum shell capacity is governed by Bohr-Bury rule:2n².
- Isotopesshare atomic number Z;Isobarsshare mass number A.
Check yourself
Question 1
Which of the following subatomic particles was discovered by bombarding beryllium with alpha particles in 1932?