PRELIMS

Atomic and Nuclear Physics

Atomic Structure and Particles

Atom: Smallest part of matter involved in chemical reactions. Consists of a nucleus (protons + neutrons) and revolving electrons.
Fundamental Particles:

Proton: Mass (kg): . Charge (C): . Discoverer: Rutherford. Notes: Named by Rutherford; Discovered by Goldstein.
Neutron: Mass (kg): . Charge (C): 0. Discoverer: Chadwick. Notes:
Electron: Mass (kg): . Charge (C): . Discoverer: J.J. Thomson. Notes:
Positron: Mass (kg): . Charge (C): . Discoverer: Anderson. Notes: Antiparticle of electron.
Neutrino: Mass (kg): 0. Charge (C): 0. Discoverer: Pauli. Notes: Unstable.
Pi-meson: Mass (kg): . Charge (C): . Discoverer: Yukawa. Notes:
Photon: Mass (kg): 0. Charge (C): 0. Discoverer: Einstein. Notes: Velocity = .

Evolution of Atomic Models

Dalton's Model (1808): Atom is the smallest, indivisible particle of matter.
Thomson's Model (1904): "Plum pudding" model — positive charge spread throughout the atom with electrons embedded like plums (did not explain nucleus).
Rutherford's Model (1911): Based on the alpha-particle scattering (gold foil) experiment. Proposed a small, dense, positively charged nucleus at the centre with electrons revolving around it; atom is mostly empty space. Could not explain stability of the atom.
Bohr's Model (1913): Electrons revolve in fixed, discrete circular orbits ("shells") of specific energy without radiating energy; energy is absorbed/emitted only when electrons jump orbits. Explained the hydrogen spectrum.
Quantum Mechanical Model: Modern model; electrons exist in probability clouds ("orbitals") rather than fixed paths.

Confused Pairs: Isotopes, Isobars, Isotones

Isotopes: Same atomic number (Z), different mass number (A) — same element, different neutron count. Example: .
Isobars: Same mass number (A), different atomic number (Z) — different elements. Example: and .
Isotones: Same number of neutrons, different atomic number and mass number. Example: and (both have 8 neutrons).

2. Cathode and Canal Rays

Cathode Rays: Discovered by Sir William Crooke and J.J. Thomson.
- Properties:
  1. Travel in straight lines with velocity of light.
  2. Produce fluorescence and affect photographic plates.
  3. Reach high penetration in thin metal foils.
  4. Deflected by electric and magnetic fields.
Canal Rays (Positive Rays): Discovered by Goldstein.
- Properties: Consist of positively charged particles; deflect in EM fields.

3. Radioactivity

Definition: Spontaneous disintegration of heavy, unstable nuclei.
Discovery: Henri Becquerel (1896); term coined by Madam Curie.
Unit: Becquerel (Bq) or Curie (Ci) ( ).
Rays Emitted:
1. Alpha ( ) Rays: Helium nuclei ( ); high ionizing power, low penetrating power.
2. Beta ( ) Rays: Fast-moving electrons; moderate ionizing and penetrating power.
3. Gamma ( ) Rays: High-frequency EM waves; maximum penetrating power, minimum ionizing power.
Soddy-Fajan Law: Rules governing changes in mass/atomic numbers during emission.
Detection: G.M. (Geiger-Müller) Counter.
Half-life: Time for half of the nuclei of a radioactive sample to decay; constant for a given isotope, independent of quantity/external conditions (temperature, pressure). Decay is exponential (never truly reaches zero).
Carbon Dating ( ): Used to age fossils/plants by ratio; half-life years.

Confused Pairs: Alpha vs Beta vs Gamma Radiation

Property	Alpha ( )	Beta ( )	Gamma ( )
Nature	Helium nucleus ( )	Fast electron	EM radiation
Charge	+2	-1	0
Ionizing Power	Highest	Moderate	Lowest
Penetrating Power	Lowest (stopped by paper)	Moderate (stopped by thin aluminium)	Highest (needs thick lead/concrete)
Effect of B-field	Deflects less (heavier)	Deflects more (lighter)	No deflection

Nuclear Fission and Controlled Reactions

Nuclear Fission: Heavy nucleus splits into nearly equal parts. Demonstrated by Strassmann and Hahn.
- Reaction: bombarded by neutrons produces , and 3 neutrons.
Chain Reaction: Fission neutrons causing further fission.
1. Uncontrolled: Atom Bomb ( or ). First used in 1945 (Hiroshima/Nagasaki).
2. Controlled: Nuclear Reactor.
Nuclear Reactor (Atomic Pile): First established by Enrico Fermi (Chicago University).
- Fuel: or .
- Moderator: Slows neutrons (Heavy Water, Graphite).
- Control Rods: Absorbs excess neutrons (Cadmium, Boron).
- Coolant: Absorbs heat (Water, Heavy Water, , or ).
- Fast Breeder Reactor: Produces more fuel than it consumes.

Nuclear Fusion

Definition: Light nuclei combining to form a heavier nucleus. Requires .
Solar Energy: Source of energy in Sun and Stars.
Hydrogen Bomb: Based on fusion. 1000x more powerful than atom bomb (USA, 1952).

Confused Pairs: Fission vs Fusion

Fission: Heavy nucleus splits into lighter nuclei; occurs at normal temperature; used in nuclear power plants and atom bombs; produces radioactive waste.
Fusion: Light nuclei combine into a heavier nucleus; requires extremely high temperature ( K); powers the Sun/stars and hydrogen bombs; releases more energy per reaction than fission and produces less long-lived radioactive waste.

Recent Developments (India's Nuclear/Fusion Research)

Three-Stage Nuclear Programme (Homi Bhabha): (1) Pressurised Heavy Water Reactors using natural Uranium, (2) Fast Breeder Reactors using Plutonium, (3) Thorium-based reactors (India has the world's largest thorium reserves).
ITER (International Thermonuclear Experimental Reactor): India is a member of this multinational fusion project being built in France, aimed at demonstrating sustained nuclear fusion.
SST-1 / Aditya-L1 context: India's Institute for Plasma Research operates the SST-1 tokamak for fusion research (note: Aditya-L1 is a solar observation mission, not a fusion reactor — do not conflate the two).

Mass-Energy Relation

UPSC Relevance

Atomic and Nuclear physics are frequently tested in Prelims S&T.

Nuclear Program: Components of a reactor (Moderators vs Control Rods) and India's 3-stage program context.
Common Trap: Fusion (Hydrogen Bomb/Stars) vs Fission (Nuclear plants/Atom bomb).
Applications: Carbon dating for archaeology and G.M. counters for radiation monitoring.

Atomic and Nuclear Physics ​