where A = Z + N {\displaystyle A=Z+N} is the total number of nucleons (Mass Number). The terms proportional to A {\displaystyle A} and A 2 / 3 {\displaystyle A^{2/3}} represent the volume and surface energy of the liquid drop, the term proportional to Z 2 {\displaystyle Z^{2}} represents the electrostatic energy, the term proportional to ( N − Z ) 2 {\displaystyle (N-Z)^{2}} represents the Pauli exclusion principle and the last term δ ( A , Z ) {\displaystyle \delta (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 {\displaystyle a_{V},a_{S},a_{C},a_{A}} and the strength of the pairing term may be estimated theoretically, or fit to data.