N(t) = N₀ × (½)^(t/t½)
Half-life (t½) is the time required for half of a substance to decay or react. For first-order processes: N(t) = N₀ × e^(−λt) = N₀ × (½)^(t/t½), where λ = ln(2)/t½ is the decay constant. After n half-lives, the fraction remaining = (½)ⁿ. Half-lives vary enormously: ²³⁸U = 4.5 billion years, ¹⁴C = 5,730 years (used in radiocarbon dating), ¹³¹I = 8 days (medical imaging), ²²⁰Rn = 56 seconds. In pharmacology, drug half-life determines dosing intervals — steady state is reached after about 5 half-lives. In chemistry, first-order reaction half-life is independent of initial concentration (t½ = ln2/k), unlike second-order reactions where t½ = 1/(k[A]₀).