where m p and m n are the rest mass of a proton and a neutron, respectively, and E B is the total binding energy of the nucleus. The mass–energy equivalence is used here. The binding energy is subtracted from the sum of the proton and neutron masses because the mass of the nucleus is less than that sum. This property, called the mass defect, is necessary for a stable nucleus; within a nucleus, the nuclides are trapped by a potential well. A semi-empirical mass formula states that the binding energy will take the form