An alpha particle is made of two protons and two neutrons. An alpha decay involves the ejection of an alpha particle from the nucleus.
Therefore since an alpha particles is equivalent to a helium nucleus, it involves the loss of 2 protons and 2 neutrons.
Hence, we can conclude that when a nucleus emits an alpha particle, the mass number or the atomic mass decreases by 4 and the atomic number decreases by 2.
