time required for half of a quantity of radioactive atoms to decay
follows exponential decay: N(t) = N0 * (1/2)^(t / t_half)
characteristic of each isotope: carbon-14 ~5730 years, uranium-238 ~4.5 billion years, iodine-131 ~8 days
used in radiometric dating of rocks, fossils, and archaeological artifacts
medical applications: isotopes with short half-lives serve as diagnostic tracers and therapeutic agents
biological half-life describes how fast a substance is cleared from the body
plasma half-life governs drug dosing schedules in pharmacology
the concept extends to any exponential decay process: chemical reactions, population decline, signal attenuation