N(t) = N₀·exp(−λt); t_{1/2} = ln 2 / λ (universal isotope clock)

Radiometric decay (Rutherford 1900; Holmes 1913 applied it to zircon for the first radiometric geochronology). The number of un-decayed parent isotopes evolves as N(t) = N_0·exp(−λt), where λ is the first-order decay constant and t_{1/2}…