Slide 11 of 13
So here we have binding energy per nucleon for all the stable nuclei plotted on one graph. This is no doubt the best quantitative measure of relative stability of the different nuclei. The plot readily shows how much energy can be obtained from various kinds of nuclear reactions. We’ll come back to it when we talk about radioactivity, bombs and nuclear power plants.
Notice that the region between Z=20 and Z=40 represents the most stable of all nuclei -- the “brick houses” of isotopes. This is the region around elements like iron, nickel and cobalt.
For less massive nuclei, the ratio of nucleons at the surface to those inside is small, and the nucleons on the surface can’t take full advantage of being fully surrounded by others (just like it’s easier to knock bricks off the corners of a house because they’re stuck to fewer other bricks).
As nuclei get larger and larger, these effects at the surface become less important than the buildup of positive charge on the nucleus.