The binding energy per nucleon is the average binding energy of a nucleon in the nucleus. The energy required to remove a nucleon from the nucleus is the separation energy. This is equal to the difference between the masses of the separated fragments and the original mass, for example the proton separation energy is given by:
General considerations of stability lead to the expectation that a system of nucleons will tend to a system of lowest potential energy (highest binding energy per nucleon). Thus one can expect if the proton or neutron separation energies become negative, it will be favourable for separation to occur. (The lines in the (N,Z) plane along which this occurs are known as `drip lines').