What determines how many neutrons can exist in a nucleus along with a given number of protons? Some combinations are stable indefinitely, some combinations are never found, and intermediate ratios of neutrons to protons lead to unstable nuclei that decay in different ways into other nuclei. Nuclei are only stable if they have as manyas, or more neutrons than protons; the only stable nucleus with a surplus of protons is . The stable region of protons and neutrons is shown in the graph on the next page. White spots indicate stable nuclei, while green, blue, red and yellow dots represent unstable nuclei that decay.

Nuclei with too many neutrons, below and to the right of the stable region in the graph, turn one neutron into a proton by emitting an electron or particle from the nucleus. This is called beta decay: