Question

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

Answer 1

In an `alpha` decay the mass number increases by 4, in a `beta` decay it is unchanged.

Explanation:

Whether or not the mass number changes depends on the type of decay.

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.