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Alpha decay (A → A−4 + ⁴He) occurs in heavy nuclei where emission of a ⁴He nucleus (alpha particle) reaches the stability curve. Alpha particles are tightly bound (high binding energy), making alpha emission energetically favorable. The Q-value α-decay = (M_parent − M_daughter − M_He)c² provides recoil kinetic energies split between daughter nucleus and alpha particle (inverse mass ratio). Alpha decay competes with beta and spontaneous fission.
From your study of radioactive decay you know that unstable nuclei spontaneously transform to release energy, and from nuclear structure you know that the binding energy per nucleon peaks around mass number A ≈ 56 (iron) and decreases for very heavy nuclei. Alpha decay is the mechanism by which heavy nuclei (typically A > 140) shed four nucleons at once to move toward greater stability. The key to understanding why the emitted fragment is specifically a helium-4 nucleus — two protons and two neutrons — lies in its exceptional stability: the alpha particle has a binding energy of 28.3 MeV, making it one of the most tightly bound light nuclei. Emitting an alpha particle is energetically much more favorable than emitting four individual nucleons or even two separate protons and two separate neutrons.
The energetics are governed by the Q-value: Q = (M_parent − M_daughter − M_α)c². When Q > 0, energy is released and the decay is spontaneous. This Q-value appears almost entirely as kinetic energy of the products, split between the alpha particle and the recoiling daughter nucleus in the inverse mass ratio. Since the daughter nucleus is much heavier (A − 4 vs. 4), it recoils with very little kinetic energy, and the alpha particle carries away nearly all of the Q-value as kinetic energy. This is why alpha particles from a given decay appear as a nearly monoenergetic beam — their energy is sharply defined by Q, unlike the continuous energy spectrum of beta particles. Alpha spectroscopy exploits this: measuring the energy of emitted alpha particles identifies the parent nucleus.
There is, however, a profound puzzle. For heavy nuclei, the Q-value is positive — the decay is energetically allowed — yet the nucleus often persists for millions or billions of years before decaying. Classically, the alpha particle is trapped inside the nucleus by a Coulomb barrier: to escape, it would need to tunnel through a potential energy barrier (the electrostatic repulsion between the alpha's +2e charge and the daughter's +Ze charge) that is several tens of MeV high, far above the alpha's kinetic energy. This is impossible classically, but quantum mechanics allows tunneling: the alpha particle has a non-zero probability of penetrating the barrier. The tunneling probability depends exponentially on the barrier height and width, which explains the enormous variation in alpha-decay half-lives (from microseconds to billions of years) that correlates with relatively small variations in alpha energy (from about 4 to 9 MeV). This exponential sensitivity — known as the Geiger-Nuttall law — was one of the first quantitative triumphs of quantum tunneling in nuclear physics.
Alpha decay produces a daughter nucleus that is often left in an excited state, explaining why alpha decay is frequently followed by gamma emission. The daughter nucleus then de-excites by emitting one or more gamma-ray photons before reaching its ground state. The complete decay chain of heavy elements — thorium, uranium, radium — consists of alternating alpha and beta decays, each step moving the nucleus closer to the valley of stability on the nuclear chart, with gamma emission accompanying many steps.