Alpha decay is a type of radioactive decay in which an unstable atomic nucleus emits an alpha particle, which consists of two protons and two neutrons (equivalent to a helium-4 nucleus). This process reduces the mass number of the original nucleus by four and the atomic number by two, resulting in the transformation of the original element into a different element.
Example:
Consider the alpha decay of uranium-238 (\(^{238}_{92}\text{U}\)). During this decay process, uranium-238 emits an alpha particle:
\[ ^{238}{92}\text{U} \rightarrow ^{234}{90}\text{Th} + ^{4}_{2}\text{He} \]
In this reaction, the uranium-238 nucleus loses an alpha particle and transforms into thorium-234 (\(^{234}{90}\text{Th}\)). The emitted alpha particle is represented by helium-4 (\(^{4}{2}\text{He}\)), which can also be simply referred to as an alpha particle. This decay process is one of the primary ways heavy nuclei become more stable.