Alpha decay is a type of radioactive decay in which an unstable atomic nucleus releases an alpha particle. This particle consists of two protons and two neutrons, which is the same as a helium nucleus. When an atom undergoes alpha decay, it loses these particles, resulting in a new element with a lower atomic number. This process decreases the mass of the original atom and transforms it into a different element. For example, when uranium-238 undergoes alpha decay, it becomes thorium-234. Alpha decay is common in heavy elements and is one of the ways that unstable nuclei seek stability.