An alpha particle (α) is a particle consisting of two protons and two neutrons and therefore carrying a charge of +2; it is identical to a helium-4 nucleus. Alpha particles are emitted by many heavy radioactive nuclei and are the least penetrating of the common forms of radiation. Compared with a beta particle, an alpha particle is far more massive and more strongly charged.

α 粒子(阿尔法粒子)是由两个质子和两个中子组成、因而带 +2 电荷的粒子;它与氦-4 原子核相同。许多重的放射性核会放出 α 粒子,它是常见辐射形式中穿透力最弱的。与 β 粒子相比,α 粒子质量大得多,所带电荷也更强。