Alpha particle is composed of two neutrons and two protons.

A type of radioactive decay during which an alpha particle is released from the nucleus of an unstable isotope. The alpha particle is in fact the nucleus of a helium atom composed of two protons and two neutrons. It has two positive charges. The atom that released the alpha particle thus lost two protons and underwent transmutation into another atom with a two-number lower atomic number and a four-number lower mass number. The new atom is more stable than the previous one. A typical example of alpha decay is of uranium 238 into thorium 234. The alpha particles have an energy of about 5 MeV and have high ionization power, but they can be shielded by a sheet of paper or by human skin.