Question

At low atomic numbers, stable atomic nuclei have roughly equal numbers of protons and neutrons. How does the neutron/proton ratio change for stable nuclei as the atomic number increases? What is the approximate n:p ration for elements near the end of the periodic table (atomic number above 80)?

Answer 1

This ratio generally increases with increasing atomic number. This is because electrical repulsive forces 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.

approximate n:p ration for elements near the end of the periodic table (atomic number above 80) = 0.6