Question

After a nucleus undergoes radioactive decay, its new mass number is:


always less than its original mass number

always more than its original mass number

never less than its original mass number

never more than its original mass number

Answer 1

Mass is from the number of protons and neutrons. Atomic number is the number of protons.
Alpha decay is loss of of two neutrons and two protons. So mass goes down four and atomic number goes down two.
In beta decay, a neutron is converted into a proton and an electron is ejected. So mass stays the same but atomic number goes up one.
Gamma decay is emission of a high energy photon as a nucleus goes from an excited state to a ground state. It involves no change in mass or atomic number.