Q value

The $\boldsymbol{Q}$ value of a nuclear decay process is the energy released by the decay. It is the difference of the rest energies before and after the decay:

where $c$ is the speed of light. Equivalently, by conservation of energy, it is the kinetic energy difference:

Be careful: the ordering of the difference is inverted compared to the rest energy. The $Q$ value is very useful to determine if the process occurs naturally: if it is positive, the process is spontaneous (exothermic), otherwise it is not (endothermic). The higher the $Q$ value is, the more energetically favored the process is, thus making it more common compared to other possible processes.