- noun a nucleus of the same composition as a helium atom with two protons and two neutrons
- A nuclear particle consisting of two protons and two neutrons and having a positive electrical charge. An alpha particle is essentially the nucleus of a helium atom, the second most abundant atom in the universe, and alphas are common in nature, especially in cosmic rays and as a product of the radioactive decay of heavier atoms.
- Positively charged helium nuclei emitted by many radioactive elements. Each particle consists of two protons and two neutrons. After a nuclei has emitted an alpha particle, it will have a lower mass. This means that a transformation has occurred that has yielded a different, and lighter, element. When an alpha particle is combined with two electrons, the complete helium atom is formed.