How is the atomic mass changed by beta decay?
This is because during beta decay a neutron decays into a proton. An electron is emitted from the nucleus - this is the beta particle.
As a result of the creation of a new proton, the atomic mass increases. Example,
If a Plutonium radioisotope were to undergo beta decay, the atomic number would change from 94 (Plutonium's atomic number) to 95 (Americium's atomic number) - Plutonium has, therefore, decayed into Americium.