alpha particle
Sign in to saveparticle of alpha radiation; helium-4 nucleus; particle of two protons and two neutrons
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An alpha particle is a tiny bundle of matter made up of two protons and two neutrons—essentially the nucleus of a helium atom—that is released when certain radioactive materials break down. It matters because understanding alpha particles helps scientists study radioactivity, nuclear reactions, and the behavior of unstable elements.
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Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to the nucleus of a helium-4 atom. They are generally produced in the process of alpha decay but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α. Because they are identical to helium nuclei, they are also sometimes written as He or 2He indicating a helium ion with a +2 charge (missing its two electrons). Once the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom 2He.
Alpha particles have a net spin of zero. When produced in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV and a velocity in the vicinity of 4% of the speed of light. They are a highly ionizing form of particle radiation, with low penetration depth (stopped by a few centimetres of air, or by the skin).