Gamma Emission

Nuclei, like atoms, can become excited. They have "energy levels".

After an alpha or beta decay the daughter nucleus is often left in an excited state. It has a packet of energy that it will get rid of by emitting a photon of gamma radiation.

There is no change in mass number or atomic number.

Here is a summary of the properties of gamma radiation;

In most decays the daughter nucleus is still unstable and will go on to decay further. We end up with a "decay chain" with one isotope changing into another then that changing also until we eventually end up with a stable isotope.