7. Nuclear Chemistry
Chemical reactions rearrange electrons; nuclear reactions rearrange the nucleus itself — releasing millions of times more energy per atom, and changing one element into another.
Types of Radioactivity
- Alpha (α): $^{4}\text{He}$ nucleus — heavy, slow, stopped by paper. Loses 2 protons and 2 neutrons.
- Beta (β⁻): an electron ejected when a neutron becomes a proton — stopped by aluminium. Atomic number increases by 1.
- Gamma (γ): high-energy light — deeply penetrating, stopped only by thick lead or concrete.
Example: Alpha decay of uranium-238: $$^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}$$
Mass numbers (238 = 234 + 4) and atomic numbers (92 = 90 + 2) both balance.
Half-Life
Radioactive decay is first-order: after each half-life, half the sample remains.
Example: Carbon-14 has $t_{1/2} = 5730$ years. A sample with one-quarter of its original $^{14}$C activity has passed through 2 half-lives: $2 \times 5730 = 11{,}460$ years old.
Fission and Fusion
Fission splits a heavy nucleus (e.g., $^{235}$U) into lighter ones, releasing energy — the basis of nuclear reactors. Fusion joins light nuclei (e.g., hydrogen into helium) — the energy source of the Sun. Both convert a small mass loss into enormous energy: $E = mc^2$.
Practice Problems
- Write the product of alpha decay of 214Po.
- Iodine-131 has a half-life of 8.0 days. What fraction remains after 24 days?
- Write the nuclear equation for the beta decay of carbon-14.
- Which releases more energy per reaction: fission or a typical chemical reaction?
Answers: 1. 210Pb 2. 1/8 3. 146C → 147N + 0−1e 4. Fission — by roughly a factor of a million