Alpha decay is a type of radioactive decay in which an unstable atomic nucleus emits an alpha particle. An alpha particle consists of two protons and two neutrons, which is essentially the same as a helium nucleus. This process reduces the atomic number of the original atom by two and the mass number by four, resulting in the formation of a new element. The theory of alpha decay is based on quantum tunneling, where the alpha particle escapes the nucleus despite being bound by strong nuclear forces. This phenomenon occurs because of the probabilistic nature of quantum mechanics, allowing particles to pass through energy barriers that they would not be able to overcome classically.