# When a nucleus undergoes radioactive decay, what is its new mass number?

In an $\alpha$ decay the mass number increases by 4, in a $\beta$ decay it is unchanged.
An $\alpha$ decay means that the nucleus loses two protons and two neutrons so its mass number (also, and probably better, called the 'nucleon number') decreases by 4.
In a $\beta$ decay a neutron decays to give a proton and an electron, so the mass number is unchanged, but the atomic number increases by 1.