The first term a is called the saturation contribution and ensures that the binding energy per nucleon is the same for all nuclei to a first approximation. The term − − b / A 1 / 3 is a surface tension effect and is proportional to the number of nucleons that are situated on the nuclear surface; it is largest for light nuclei. The term − − c Z 2 / A 4 / 3 is the Coulomb electrostatic repulsion; this becomes more important as Z increases. The symmetry correction term − − d ( N − − Z ) 2 / A 2 takes into account the fact that in the absence of other effects the most stable arrangement has equal numbers of protons and neutrons; this is because the n–p interaction in a nucleus is stronger than either the n−n or p−p interaction. The pairing term ± ± e / A 7 / 4 is purely empirical; it is + for even–even nuclei and − for odd–odd nuclei. When A is odd, the pairing term is identically zero.