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The nucleus of an atom is built from protons and neutrons, themselves composites of quarks. Each element has a fixed proton count, but isotopes differ in neutron number. When a nucleus can reach a lower‑energy configuration, it may transform into a different element.
Quantum mechanics tells us that an unstable nucleus will eventually shed energy, but it cannot predict the exact moment of decay for any single atom. Instead, it provides a half‑life: the average time over which a large group of identical nuclei will decay. The first three decay modes identified—alpha, beta, and gamma—form the backbone of radioactive decay.
Alpha decay ejects a helium nucleus (two protons and two neutrons). For example, uranium‑238 (92 p + 146 n) emits an alpha particle to become thorium‑234 (90 p + 144 n). Beta decay converts a neutron into a proton, emitting an electron and an antineutrino. Carbon‑14 (6 p + 8 n) undergoes beta decay to nitrogen‑14 (7 p + 7 n).
After alpha or beta emission the daughter nucleus often remains in an excited state. To reach its ground state, the nucleus releases the surplus energy as a gamma ray—an electromagnetic photon with a frequency far higher than visible light. Gamma rays travel at light speed and carry only energy, no charge or mass. A classic case is cobalt‑60, which beta‑decays to nickel‑60 and then emits two gamma photons as it settles to its lowest energy level.
Most excited nuclei emit gamma rays almost instantaneously, but some are “metastable,” holding the excess energy for times ranging from fractions of a second to many years—when a change in nuclear spin blocks immediate gamma emission. When a surrounding electron absorbs a gamma photon, the electron may be ejected from its orbit in the photoelectric effect, illustrating the intimate link between nuclear and atomic processes.
