Elements from the island of stability

The nucleus of each atom is made up of protons with the addition of more or less neutrons. It is thus theoretically possible for an element to exist with any number of protons in its nucleus. Perhaps a thousand or a million. But protons, as charged particles, repel each other, and the more there are in the nucleus, the harder it is to keep the nucleus together. This is partly helped by the neutrons, which act as a glue. The short-range nuclear forces between them and the protons hold the atomic nucleus together.

Even so, the larger the nucleus, the less stable it tends to be and sooner or later it will break apart. Nuclei that have half-lives of a million years or more are considered to be stable enough. The heaviest stable element found in significant quantities on Earth is uranium, with a proton number of 92. Artificially produced Curium with a proton number of 96, whose isotope 247Cm has a half-life of sixteen million years, could also be considered sufficiently stable. However, heavier elements have half-lives of years, days, and the heaviest elements of only fractions of seconds.

Moreover, the stability of the nucleus strongly depends on the number of neutrons. One isotope may be stable, but the addition or removal of a neutron turns it into a strongly unstable isotope.

The theory of magic numbers tries to explain why this is so. According to it, the nucleus of an atom is arranged like an electron shell, with energy shells that neutrons and protons fill. An atom that has one entire shell filled is more stable than others. The numbers of neutrons and protons that lead to a filled shell are called magic numbers. If an atom contains both magic numbers of protons and neutrons, it is called "doubly magic" and should be that much more stable.

Plotting the half-lives of individual isotopes against their neutron and proton counts, we get something like a long mountain range of stability, surrounded by shallow shelves of relative stability, around which is a wide ocean of instability with isotopes with extremely short half-lives. But with a proton number above 90, the continent of stability is slowly sinking into the waters of instability.