ABSTRACT
A very short-range attraction acts as the "glue" to bind protons and neutrons into a nucleus. This nuclear force is only effective over distances of about 10-15 m, which therefore must represent the approximate diameter of the nucleus. The number of protons in an element's nucleus is the atomic number and the sum of the protons and neutrons is the mass number. Nuclei with equal atomic numbers but different mass numbers are isotopes of one another – they are the same element, but have different numbers of neutrons. The nuclear force is quite strong – so strong that by comparison the electrostatic repulsion between the protons is negligible. Certain naturally occurring isotopes of some elements are able to blacken photographic paper merely by being stored in the dark near the paper. Evidently the isotopes are spontaneously releasing some kind of radiation; they are said to be "radioactive".
