📑 Topics on this page (Jump to Topic):
📌 1. Fundamental Particles
| Particle | Discoverer | Charge | Mass |
|---|---|---|---|
| Electron | J.J. Thomson (1897) | −1 | ≈ 1/1836 of proton |
| Proton | Rutherford (1919) [Goldstein — canal rays] | +1 | ≈ 1 u |
| Neutron | James Chadwick (1932) | 0 | ≈ 1 u (slightly more than proton) |
| Nucleus | Rutherford (1911, gold-foil experiment) | + | Almost all the atom's mass |
- The electron's charge was measured by Millikan (oil-drop experiment). Ordinary hydrogen (protium) has no neutron.
- Dalton (1808): atom indivisible — the first atomic theory. The idea of the atom (parmanu) was given in ancient India by Maharshi Kanad.
📌 2. Atomic Models
| Model | Key idea | Drawback |
|---|---|---|
| Thomson — "plum pudding / watermelon" | Electrons embedded in a positive sphere | No nucleus |
| Rutherford — nuclear | Tiny dense positive nucleus; atom mostly empty | Could not explain stability |
| Bohr (1913) | Electrons in fixed energy orbits (K, L, M, N) | Fails for multi-electron atoms |
| Quantum mechanical | Schrödinger, Heisenberg; orbitals (s, p, d, f) | — |
📌 3. Numbers, Isotopes & Electronic Configuration
- Atomic number (Z) = number of protons (= electrons in a neutral atom). Mass number (A) = protons + neutrons. Neutrons = A − Z.
- Isotopes: same Z, different A — hydrogen has three: protium (¹H), deuterium (²H, in heavy water D₂O), tritium (³H, radioactive). C-12/C-14, U-235/U-238.
- Isobars: same A, different Z — ⁴⁰Ar and ⁴⁰Ca. Isotones: same number of neutrons.
- Maximum electrons in a shell = 2n² (K=2, L=8, M=18, N=32); outermost shell max 8 (octet). E.g. Na (11) = 2, 8, 1; Cl (17) = 2, 8, 7; Ca (20) = 2, 8, 8, 2.
- Valency = outer electrons (1–4) or 8 − outer electrons. Na = 1, Mg = 2, Al = 3, C = 4, N = 3, O = 2, Cl = 1; noble gases = 0.
| Concept | Value / meaning |
|---|---|
| Avogadro number | 6.022 × 10²³ |
| 1 mole | 6.022 × 10²³ particles; molar mass in grams |
| Volume of 1 mol gas at STP | 22.4 L |
| Molar mass of water (H₂O) | 18 g/mol |
| CO₂ / NaCl | 44 / 58.5 g/mol |
⚠️ Exam Traps:
- Atomic number is the number of protons, not neutrons.
- Neutron — Chadwick; electron — Thomson.
- Heavy water contains deuterium, not tritium.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Who discovered the electron? Practice
Correct answer: (A)
In 1897 through cathode-ray experiments.
In 1897 through cathode-ray experiments.
Q2. Who discovered the neutron? Practice
Correct answer: (C)
In 1932; Nobel Prize 1935.
In 1932; Nobel Prize 1935.
Q3. The atomic nucleus was discovered by: Practice
Correct answer: (D)
1911 gold-foil (α-particle scattering) experiment.
1911 gold-foil (α-particle scattering) experiment.
Q4. Atomic number equals the number of: Practice
Correct answer: (C)
Z = number of protons; it identifies the element.
Z = number of protons; it identifies the element.
Q5. Isotopes have the same: Practice
Correct answer: (B)
Same protons, different neutrons.
Same protons, different neutrons.
Q6. The chemical formula of heavy water is: Practice
Correct answer: (D)
D = deuterium (²H).
D = deuterium (²H).
Q7. Which isotope of hydrogen is radioactive? Practice
Correct answer: (A)
³H has 1 proton and 2 neutrons; a β-emitter.
³H has 1 proton and 2 neutrons; a β-emitter.
Q8. Which particle carries no charge? Practice
Correct answer: (B)
The neutron is neutral.
The neutron is neutral.
Q9. The maximum number of electrons in the M shell is: Practice
Correct answer: (C)
2n² = 2 × 3² = 18.
2n² = 2 × 3² = 18.
Q10. The electronic configuration of sodium (Z = 11) is: Practice
Correct answer: (A)
K = 2, L = 8, M = 1; valency 1.
K = 2, L = 8, M = 1; valency 1.
Q11. The value of the Avogadro number is: Practice
Correct answer: (D)
One mole contains this many particles.
One mole contains this many particles.
Q12. Who measured the charge of the electron? Practice
Correct answer: (D)
Oil-drop experiment (1909).
Oil-drop experiment (1909).
Q13. The "plum pudding" model of the atom was given by: Practice
Correct answer: (D)
Electrons embedded in a positive sphere.
Electrons embedded in a positive sphere.
Q14. An atom has mass number 23 and atomic number 11. Its neutrons are: Practice
Correct answer: (A)
n = A − Z = 23 − 11 = 12.
n = A − Z = 23 − 11 = 12.
Q15. ⁴⁰Ar and ⁴⁰Ca are: Practice
Correct answer: (B)
Same mass number, different atomic numbers.
Same mass number, different atomic numbers.
Q16. The volume of 1 mole of gas at STP is: Practice
Correct answer: (C)
For all ideal gases at standard temperature and pressure.
For all ideal gases at standard temperature and pressure.
Q17. How many moles are there in 36 g of water? Practice
Correct answer: (B)
Moles = 36/18 = 2.
Moles = 36/18 = 2.
Q18. The valency of chlorine (Z = 17) is: Practice
Correct answer: (A)
Configuration 2, 8, 7; gains one electron to complete octet — valency 1.
Configuration 2, 8, 7; gains one electron to complete octet — valency 1.
Q19. According to Bohr's model, electrons: Practice
Correct answer: (B)
Energy is emitted or absorbed only on changing orbits.
Energy is emitted or absorbed only on changing orbits.
Q20. Who proposed the atomic (parmanu) theory in ancient India? Practice
Correct answer: (C)
Kanad, founder of the Vaisheshika school.
Kanad, founder of the Vaisheshika school.