 The stability of nuclei is often depicted graphically on a SegGray chart. Such a chart is named after Emilio SegGray, who was a famous nuclear physicist who won the 1959 Nobel Prize in Physics for his discovery of the antiproton. The SegGray chart is a way to depict all nuclei by plotting them as a function of their number of protons, z, and the number of neutrons, n. Let us consider what nuclei are stable and where they lie on this chart. The simplest stable nucleus is that from a hydrogen atom, that is a single proton. We write this as h with a superscript one, and it appears down in the lower corner of the SegGray chart with z equals one and n equals zero. h, of course, is the elemental symbol for hydrogen, which by definition has one proton. The superscript one means that this nucleus has one nucleon, in this case a single proton. Now there is another form of stable hydrogen, what we call another isotope of hydrogen. This is denoted hydrogen two, where the nucleus has one neutron and one proton. Being hydrogen, of course, it must have one proton, but the superscript two allows us to infer that this is a different form of hydrogen with one proton and one neutron, giving two nucleons overall. We can place it on the SegGray chart at z equals one and n equals one. Now let us look at a more complex nucleus, calcium 40. All calcium nuclei have 20 protons, but the isotope of calcium that is labelled calcium 40 must have 20 protons and 20 neutrons for 40 nucleons overall. Calcium 40 is also stable, and we can put it on the SegGray chart at z equals 20 and n equals 20. Experimentally it is found that calcium 40 is the stable calcium isotope with the smallest number of nucleons. The heaviest stable calcium isotope is calcium 48, which has 20 protons and 28 neutrons. It appears on the SegGray chart at z equals 20 and n equals 28. You might ask why this nucleus can have so many extra neutrons. The reason is that the 20 protons all repel each other and try to break the nucleus apart. The presence of the extra neutrons allows the attractive strong nuclear force that acts between all the nucleons to hold the nucleus together. For very heavy nuclei, with a large number of protons repelling each other due to the Coulomb force, many extra neutrons are needed to hold the nucleus together and make it stable. For example, one of the heaviest stable nuclei is lead 208 with 82 protons and 126 neutrons. For light nuclei, the line of stable nuclei roughly follows the diagonal along n equals z with equal numbers of protons and neutrons. But for heavy nuclei, with nucleon numbers above 80 to 100, the line of stability eventually bends over so that the heavy stable nuclei have n much greater than z, that is more neutrons than protons. In this video we have concentrated on stable nuclei. The next video will discuss the behavior of unstable nuclei.