(it is also plotted in Weidner and Sells, Evans, and Sorenson and Phelps this way. It is also useful to plot Z vs. N, which allows us to directly compare the stable nuclides with the radionuclides on a standard "chart of the nuclides" (to be discussed later). The Z versus N plot of stable nuclides:
The most important feature to note about the line of stability is that more neutrons are required for nuclei to remain stable as the atomic number is increased. The simple explanation for this phenomenon is that more strong nuclear force is needed to compensate for the increasing Coulomb repulsion in the larger nuclei. In terms of the shell model, a kind of nuclear Pauli principle is applied to successively fill nuclear energy levels or shells, analogously to the filling of electron shells.