Chemistry · Ch 13 — Nuclear Chemistry and Radioactivity
Neutron to Proton Ratio (N/Z)
Neutron to Proton Ratio (N/Z)
A second major factor governing nuclear stability is the neutron-to-proton ratio, . Plotting the neutron number against the proton number for every known stable nuclide does not give a scattered cloud of points -- it traces out a narrow, well-defined diagonal band called the belt (or zone) of stability. Every stable nuclide falls inside this belt; any nuclide whose combination falls outside it is radioactive.
For light nuclei -- lighter than 40-20-Ca -- the stability belt sits almost exactly on the reference line , meaning light stable nuclides have close to equal numbers of protons and neutrons, i.e. . …
What this figure shows. A scatter plot with the number of protons Z on the horizontal axis (0 to about 100) and the number of neutrons N on the vertical axis (0 to about 230). The stable nuclides, plotted as points, cluster into a narrow diagonal band called the stability zone or stability belt -- any nuclide falling outside this belt is radioactive. A straight reference line marked N = Z runs through the belt for light nuclides. For nuclei lighter than 40-20-Ca, the stability belt tracks this N = Z line closely, i.e. light stable nuclides have N/Z nearly equal to 1. For nuclides heavier than calcium, the belt curves upward and away from the N = Z line, so N/Z grows steadily above 1 -- heavier nuclei need proportionally more neutrons than protons to stay stable, because as proton number increases, the coulombic (electrostatic) repulsion between the many protons grows large, and additional neutrons help keep the protons further apart on average, providing extra attractive nuclear force to offset that repulsion. Separate marker styles distin …