The radioactive decay of an atomic nucleus by emission of an alpha particle. Type of radioactive disintegration (see radioactivity) in which some unstable atomic nuclei dissipate excess energy by spontaneously ejecting an alpha particle. Alpha particles have two positive charges and a mass of four atomic mass units; they are identical to helium nuclei. Though they are emitted at speeds about one-tenth that of light, they are not very penetrating and have ranges in air of about 1-4 in. (2.5-10 cm). Alpha decay commonly occurs in elements with atomic numbers greater than 83 (bismuth), but can occur in some rare-earth elements in the atomic-number range of 60 (neodymium) to 71 (lutetium). Alpha decay half-lives range from about a microsecond (10^-6 second) to billions of years (10^17 seconds)
The process of radioactive decay in which the nucleus of an atom emits an alpha particle The new atom's atomic number is lower by two and its atomic mass number is reduced by four
The emission of an alpha particle by the nucleus The atomic number of the parent nucleus is decreased by two and the atomic weight is decreased by four