Chart of nuclides, c 1984.
© Science Museum / Science & Society Picture Library
Detail. A nuclide is an atom described in terms of the composition of its nucleus; in other words the number of protons and neutrons it contains. Different nuclides of each element can occur with different numbers of neutrons. Although they are chemically indistinguishable, the number of neutrons present give these nuclides have different atomic weights. This type of chart, of which the image shows only a part, shows the posible nuclides of each chemical element, with each horizontal row representing one element. The colours refer to the half life, or rate of radioactive decay of the nuclides. The charts are updated frequently as research continually reveals new nuclides, some of them extremely short-lived.