In this type of radioactive decay an alpha particle, consisting of two protons and two neutrons, is emitted from the parent nuclide.
The alpha particle has a mass number of four and an atomic number of two, by far the heaviest of all the types of decay! It is sometimes referred to as a helium nucleus as they look exactly the same.
Since the parent nuclide loses an alpha particle, the daughter nuclide must be two protons and two neutrons smaller than its parent. Because of this, its mass number is reduced by four and its atomic number by two.
Nuclides that emit alpha particles are very useful! We use them to help detect fires with smoke detectors, treat cancerous tumors, and even as power sources on spacecraft!