The mass number of an element is NOT changed when it emits

Radioactive decay involves the emission of particles or energy from an unstable nucleus. In alpha decay, the nucleus loses two protons and two neutrons, which causes the mass number to decrease by four. Conversely, beta decay involves the transformation of a neutron into a proton (beta-minus) or a proton into a neutron (beta-plus); since both protons and neutrons have a mass of approximately 1 and the emitted electron/positron has negligible mass, the total mass number remains unchanged. Gamma radiation consists of high-energy electromagnetic waves or photons. Because gamma rays have no mass or charge, their emission allows the nucleus to transition from an excited state to a lower energy state without altering the atomic number or the mass number. Therefore, the mass number is not changed during beta and gamma radiations.