When a nucleus emits radiation, its mass number or its charge (atomic number), or both, can change. The nucleus then becomes a different nucleus, and often a different element.
In alpha emission the nucleus loses 2 protons and 2 neutrons. Its mass number falls by 4 and its atomic number falls by 2. In beta emission a neutron in the nucleus changes into a proton and an electron, and the electron is emitted. The mass number stays the same because an electron has almost no mass, but the atomic number increases by 1.
Gamma rays carry away energy only. A nucleus often emits a gamma ray after an alpha or beta emission, and its mass number and atomic number do not change. If a nucleus emits a neutron, its mass number falls by 1 and its atomic number stays the same, so it becomes another isotope of the same element.
