What holds the nucleus together? The nucleus is tiny, so the protons are all very close together. The gravitational force attracting them to each other is much smaller than the electric force repelling them, so there must be another force keeping them together. This other force is known as the strong nuclear force; it works only at small distances. The strong nuclear force is a very strong attractive force for protons and neutrons separated by a few femtometers, but is basically negligible for larger distances.

The tug-of-war between the attractive force of the strong nuclear force and the repulsive electrostatic force between protons has interesting implications for the stability of a nucleus. Atoms with very low atomic numbers have about the same number of neutrons and protons; as Z gets larger, however, stable nuclei will have more neutrons than protons. Eventually, a point is reached beyond which there are no stable nuclei: the bismuth nucleus with 83 protons and 126 neutrons is the largest stable nucleus. Nuclei with more than 83 protons are all unstable, and will eventually break up into smaller pieces; this is known as radioactivity.