Half-Life
The time required for half the radioactive nuclei in a given sample to undergo spontaneous radioactive decay.
Governing formula
t_½ = ln(2) / λ ≈ 0.693 / λ, N(t) = N₀ (½)^(t / t_½)
SI unit
Seconds, Hours, or Years
In depth
Half-life is a fundamental characteristic constant for each specific radioisotope, completely independent of initial sample quantity, ambient temperature, pressure, or chemical bonding state. It governs exponential radioactive decay and determines isotope stability.
Examples in the real world
Carbon-14 having a half-life of 5,730 years used in radiocarbon dating, or Uranium-238 having a half-life of 4.47 billion years.