Radioactive decay (radioactivity) is the spontaneous transformation of an unstable atomic nucleus into a more stable configuration through emission of particles or energy: alpha decay ($\alpha$), beta decay ($\beta^-$, $\beta^+$), or gamma emission ($\gamma$). It follows first-order kinetics: $N(t) = N_0 e^{-\lambda t}$, where $\lambda$ is the decay constant and $t_{1/2} = \ln 2 / \lambda$. Radiometric dating methods (U-Pb, K-Ar, Rb-Sr) rely on known decay constants.