What type of radioactive decay reduces the mass number of a nucleus? Explain how it works.

Alpha radiation reduces the mass number of an atom by 4. When an atom's nucleus is unstably large, it emits an alpha particle which consists of two neutrons and two protons. This is called alpha decay. Since both neutrons and protons have a mass of 1, the mass of the nucleus is reduced by the number of neutrons and protons that were emitted, which is 4.