How does nuclear decay affect the atomic number and mass number?
For example, the alpha decay of uranium-235 changes both the mass number and the atomic number of the element from 235 to 231 and from 92 to 90. The mass number decreases by 4, and the atomic number decreases by 2. This gives rise to the formation of a new element - thorium-231.
If the same element were to undergo beta decay, the mass number would stay the same, and the atomic number would increase by 1, giving neptunium-235.
Gamma decay is the emission of a photon from the nucleus of the atom and since gamma rays are electromagnetic waves, they are pure energy, and have no mass or atomic number. This means that the atomic and mass numbers of the atom do not change, and instead, this mode of decay brings the atom down to a less excited state, as it has less energy.