The neutron–proton ratio (N/Z ratio or nuclear ratio) of an atomic nucleus is the ratio of its number of neutrons to its number of protons. Among stable nuclei and naturally-occurring nuclei, this ratio generally increases with increasing atomic number. This is because electrical repulsive foces between protons scale with distance differently than strong nuclear force attractions. In particular, most pairs of protons in large nuclei are far enough apart that electrical repulsion dominates over the strong nuclear force, and thus proton density in stable larger nuclei must be lower than in stable smaller nuclei where more pairs of protons have appreciable short-range nuclear force attractions.