The binding energy expression gives a quantitative estimate for the neutron-proton ratio. The energy is a quadratic expression in Z that is minimized when the neutron-proton ratio is N / Z ≈ ≈ 1 + a C 2 a A A 2 / 3 . This equation for the neutron-proton ratio shows that in stable nuclides the number of neutrons is greater than the number of protons by a factor that scales as A 2 / 3 .