Nuclear Chemistry: Decay, Fission, and Fusion
Alpha, beta, gamma — the three classic decay types. Half-life problems, balancing nuclear equations, and the difference between fission (splitting heavy nuclei) and fusion (combining light nuclei). Plus how radioisotopes serve medicine and dating.
Three classic decay modes
Some nuclei are unstable and spontaneously break down, emitting radiation. The three main types:
| Type | Symbol | What it is | Change to parent |
|---|---|---|---|
| Alpha (α) | ⁴₂He | Helium-4 nucleus (2p + 2n) | Mass −4, atomic # −2 |
| Beta (β⁻) | ⁰₋₁e | High-speed electron | Mass same, atomic # +1 |
| Gamma (γ) | γ | High-energy photon | No mass or # change |
Beta decay mechanism: a neutron converts to a proton + emitted electron (the beta particle). So mass number stays the same (n becomes p, still 1 nucleon), but atomic number increases by 1.
Penetrating power (least to most)
Alpha < Beta < Gamma.
- Alpha: stopped by paper or skin
- Beta: stopped by aluminum foil or thick clothing
- Gamma: requires thick lead or concrete
But — alpha is the MOST DAMAGING if ingested or inhaled, because it deposits all its energy in a small region of tissue. External alpha is harmless; internal alpha is dangerous.
Balancing nuclear equations
Conserve TWO things on both sides:
- Total mass number (top numbers)
- Total atomic number (bottom numbers)
Example — uranium-238 alpha decay:
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Check: mass 238 = 234 + 4 ✓; atomic 92 = 90 + 2 ✓.
Example — carbon-14 beta decay:
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Check: mass 14 = 14 + 0 ✓; atomic 6 = 7 + (−1) ✓.
Half-life — radioactive decay rate
The half-life is the time required for HALF of a radioactive sample to decay. It is constant for each isotope — independent of temperature, pressure, chemical state, or sample size.
Tracking decay: starting with N atoms, after n half-lives you have
remaining = N × (1/2)ⁿ
Half-lives range enormously:
- Polonium-214: 164 microseconds
- Iodine-131: 8 days (used in thyroid medicine)
- Carbon-14: 5,730 years (used for radiocarbon dating)
- Uranium-238: 4.5 billion years
Example: A 100 g sample with half-life 10 years. After 30 years (= 3 half-lives), the remaining = 100 × (1/2)³ = 100 × 1/8 = 12.5 g.
Radiometric dating
The constant half-life makes radioactive isotopes useful as clocks. Living organisms exchange carbon with their environment, maintaining a constant ratio of C-14 to C-12. When the organism dies, C-14 starts decaying without replenishment. By measuring the remaining C-14, scientists can estimate how long ago death occurred — useful for organic samples up to ~50,000 years (≈10 half-lives).
For older samples (rocks), longer half-life isotopes are used: potassium-40, uranium-238, rubidium-87.
Fission vs fusion
Fission — a HEAVY nucleus splits into smaller fragments + neutrons + tremendous energy. Used in nuclear power plants and atomic bombs. Common fuel: uranium-235 absorbs a neutron, becomes unstable, splits.
One U-235 fission releases ~200 MeV — about a million times more energy per atom than a typical chemical reaction. The released neutrons can split more U-235, creating a chain reaction. Control rods (boron, cadmium) absorb excess neutrons to keep the reaction stable.
Fusion — LIGHT nuclei (like hydrogen) combine into a heavier one + energy. Powers stars including our Sun (~10⁷ kg of hydrogen fused per second). Fusion releases even more energy per gram than fission, but requires extreme temperatures (millions of Kelvin) to overcome electrostatic repulsion. Hydrogen bombs use fusion; controlled fusion for power is still experimental.
Both fission and fusion release energy because the products have slightly less mass than the reactants — the missing mass is converted to energy by Einstein's E = mc².
Real-world applications
- Medicine: I-131 (thyroid), Tc-99m (PET scans, bone imaging), Co-60 (cancer radiation therapy)
- Industry: Smoke detectors (Am-241 alpha source), thickness gauges, food sterilization
- Power: nuclear reactors using U-235 or Pu-239
- Dating: C-14 for organic, K-40 and U-238 for rocks