where A = Z + N is the total number of nucleons (Mass Number). The terms proportional to A and A 2 / 3 represent the volume and surface energy of the liquid drop, the term proportional to Z 2 represents the electrostatic energy, the term proportional to ( N − − Z ) 2 represents the Pauli exclusion principle and the last term δ δ ( A , Z ) is the pairing term, which lowers the energy for even numbers of protons or neutrons. The coefficients a V , a S , a C , a A and the strength of the pairing term may be estimated theoretically, or fit to data. This simple model reproduces the main features of the binding energy of nuclei.