| | Plotting a graph | | | | | | We can plot a graph of stable isotopes. The number of protons (or atomic number, Z) is on the x-axis and the number of neutrons (N) is on the y-axis. The graph is a curve - see picture 3.1. For light elements (the bottom left of the graph), the stable isotopes are the ones with the same numbers of protons and neutrons and hence the ‘line of stability’ follows the straight line of N = Z. For example, the stable isotope carbon-12 has 6 protons and 6 neutrons. | | | | | Heavier elements | | | | | | Stable isotopes of the heavier elements (top right of the graph) have more neutrons than protons. For example, Gold-197 is stable. It contains 79 protons and 118 neutrons. Hence the line begins to curve upwards, away from N = Z. | | | | | Nuclear glue | | | | | | The neutrons in a nucleus can be thought of as acting as a kind of glue to hold the nucleus together. The positively charged protons are in a very confined space and would rather not be, since they repel each other by the electromagnetic interaction. However, protons and neutrons are all attracted to each other due to another force (the strong nuclear force), and hence having more neutrons around can help to hold the nucleus together. Notice that no amount of neutrons can hold a nucleus together once it has more that 82 protons – the line stops at Z = 82! All of the elements with an atomic number greater than 82 have only unstable isotopes. | | | | |