Atomic and Nuclear Physics
~10 min read
- Atomic models: Thomson (plum pudding), Rutherford (nuclear), Bohr (orbits, quantised), modern (quantum mechanics).
- Radioactivity: Alpha (₂⁴He), Beta (electron), Gamma (γ photons). Half-life = time for half of nuclei to decay.
- Fission vs Fusion: Fission: heavy nucleus splits (atomic bomb, nuclear reactor). Fusion: light nuclei combine (sun, hydrogen bomb).
Atomic and nuclear physics underlies modern technology — nuclear power, medical imaging, smoke detectors. NDA tests atomic models, radioactivity, isotopes, fission/fusion, and half-life.
Atomic Structure
- Subatomic particles: Proton (positive, ~1 u mass), Neutron (neutral, ~1 u), Electron (-1 charge, ~1/1836 u).
- Atomic number (Z): Number of protons. Determines element.
- Mass number (A): Protons + Neutrons.
- Isotopes: Same Z, different A. e.g., ¹H, ²H (deuterium), ³H (tritium). All hydrogen but different masses.
- Isobars: Same A, different Z. e.g., ⁴⁰Ar and ⁴⁰K.
- Isotones: Same number of neutrons.
Evolution of Atomic Models
| Year | Scientist | Model |
|---|---|---|
| 1803 | Dalton | Atoms indivisible |
| 1897 | J.J. Thomson | Plum pudding — sphere of positive charge with electrons embedded |
| 1909-11 | Rutherford | Gold foil experiment. Nuclear model — small dense positive nucleus, electrons orbiting |
| 1913 | Bohr | Quantised orbits — electrons in fixed shells; emit/absorb photons when jumping |
| 1920s onwards | Schrödinger, Heisenberg, Born | Quantum mechanical model — electrons as probability clouds |
Radioactivity
- Discovered 1896 by Henri Becquerel. Marie and Pierre Curie isolated radium and polonium (1898).
- Three types of decay:
- Alpha (α): Emission of helium nucleus ₂⁴He. Z decreases by 2, A by 4. Heavy nuclei (uranium, radium).
- Beta (β): Emission of electron. A neutron converts to proton + electron + antineutrino. Z increases by 1, A same.
- Gamma (γ): Electromagnetic radiation (photons). No change in Z or A. Highest penetrating power.
- Penetrating power: α < β < γ.
- Half-life (t½): Time for half of radioactive nuclei to decay.
- Uranium-238: 4.5 billion years.
- Carbon-14: 5,730 years (used in radiocarbon dating).
- Iodine-131: 8 days (medical use).
Nuclear Fission and Fusion
- Nuclear fission:
- Heavy nucleus splits into smaller nuclei + neutrons + energy.
- Discovered by Hahn, Strassmann (1938) and explained by Meitner.
- Example: U-235 + n → Ba-141 + Kr-92 + 3n + energy.
- Chain reaction: emitted neutrons trigger further fissions.
- Applications: Atomic bombs (Hiroshima, Nagasaki 1945), nuclear power plants.
- Nuclear fusion:
- Light nuclei combine to form heavier nucleus + energy.
- Sun's energy source: fusion of hydrogen → helium.
- Requires extreme temperatures (~10⁷ K) — confinement very difficult.
- Hydrogen bomb (1952 first test) uses fusion.
- Controlled fusion (ITER project) — clean energy goal.
- Mass-Energy equivalence: E = mc². Small mass converts to huge energy.
Applications
- Nuclear power: India has Tarapur, Kalpakkam, Kudankulam, Narora, Kakrapar, Rawatbhata, Kaiga reactors.
- Radiocarbon dating: C-14/C-12 ratio in ancient organic remains tells age (good up to ~50,000 years).
- Medical imaging: X-rays, CT scans, PET, radioactive tracers (Tc-99m, I-131 for thyroid).
- Cancer treatment: Radiation therapy uses gamma from Co-60 or particle beams.
- Smoke detectors: Americium-241 alpha source.
- Sterilisation: Gamma irradiation of medical equipment.
NDA PYQ Examples
Q: Which has the maximum penetrating power?
(a) Alpha (b) Beta (c) Gamma (d) All same
Answer: (c) Gamma — electromagnetic radiation.
Q: Half-life of Carbon-14 is approximately:
(a) 50 years (b) 500 years (c) 5,730 years (d) 50,000 years
Answer: (c) 5,730 years — used in radiocarbon dating.
Q: Energy from the Sun comes from:
(a) Burning (b) Nuclear fission (c) Nuclear fusion (d) Chemical reaction
Answer: (c) Nuclear fusion — hydrogen to helium.
Q: Atomic number of an element is the number of:
(a) Protons (b) Neutrons (c) Electrons (d) Nucleons
Answer: (a) Protons.
Drill Atomic and Nuclear Physics for NDA
NDA-pattern items on Atomic and Nuclear Physics with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
What is the difference between fission and fusion?
Fission — splitting of heavy nuclei (uranium, plutonium) into smaller ones, releasing energy. Used in nuclear reactors and atomic bombs. Fusion — joining of light nuclei (hydrogen → helium) at extreme temperatures. Powers the sun and hydrogen bombs. Fusion releases more energy per kg than fission.
Why is U-235 used in nuclear reactors?
U-235 is fissile — it can undergo fission with slow (thermal) neutrons. U-238 (more abundant) is fissionable only with fast neutrons. Natural uranium is mostly U-238 (~99.3%) with ~0.7% U-235 — enriched for reactor fuel.
What is radiocarbon dating?
Living organisms continuously exchange carbon with environment. After death, C-14 (radioactive) decays with half-life 5,730 years; C-12 stays. Measuring the C-14/C-12 ratio in remains tells how long ago the organism died. Effective up to ~50,000 years.
Why is Bohr's model considered an improvement over Rutherford's?
Rutherford explained the nucleus but couldn't explain why electrons don't spiral into it (classical physics predicted they should). Bohr quantised electron orbits — said electrons can only exist in certain allowed energy levels, and emit/absorb photons when jumping between them.
How does a nuclear reactor produce electricity?
Controlled nuclear fission generates heat. Heat boils water → steam → drives turbines → produces electricity. Control rods (cadmium, boron) absorb excess neutrons to control reaction speed. Cooling systems prevent overheating.