According to Lilley, "The binding energy of a nucleus B is the energy required to separate it into its constituent neutrons and protons." m ( A , Z ) = Z m H + N m n − − B / c 2 where A is mass number, Z is atomic number, m H is the atomic mass of a hydrogen atom, m n is the mass of a neutron, and c is the speed of light. Thus, the mass of an atom is less than the mass of its constituent protons and neutrons, assuming the average binding energy of its electrons is negligible. The binding energy B is expressed in energy units, using Einstein's mass-energy equivalence relationship. The binding energy also provides an estimate of the total energy released from fission.