Nuclear Chemistry
Nuclear change
Nuclear reactions alter the nucleus and often change the element. Alpha decay emits a helium nucleus (⁴₂He), decreasing mass number by 4 and atomic number by 2. Beta-minus decay converts a neutron to a proton, emitting an electron; mass number stays the same and atomic number rises by 1. Beta-plus decay/electron capture lowers atomic number by 1. Gamma emission releases energy without changing A or Z. Nuclear equations conserve mass number and charge.
Half-life and energy
Half-life is the time for half a sample to decay. After n half-lives, remaining mass = initial mass × (1/2)^n. Radioactive decay is first-order. Mass defect converts to binding energy via E = mc². Fission splits heavy nuclei; fusion combines light nuclei and powers stars. Radiation safety uses time, distance, and shielding.
Worked example
A 10.0 g sample has a half-life of 5.0 days. After 15.0 days, n = 15.0/5.0 = 3. Remaining mass = 10.0 g × (1/2)^3 = 10.0 × 1/8 = 1.25 g. The other 8.75 g has decayed, though it is not destroyed-it becomes decay products and energy.
Traps
Thinking half-life means all material is gone after two half-lives; changing mass number in beta decay; forgetting charge conservation in nuclear equations; and treating fission and fusion as the same process.