The number of protons in a nucleus defines an element, but different numbers of neutrons lead to different isotopes of the same element. We encountered several isotopes of uranium, bismuth, and lead in the two uranium decay schemes just outlined. As another example, carbon has seven isotopes, which have been observed either in nature or in nuclear reactions.

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Only two of the seven isotopes of carbon are stable; the others break down spontaneously with different rapidity. The unstable nuclei that are rich in protons, carbon-10 and carbon-11, decay by capturing an electron or ejecting a positron. The nuclei that are oversupplied with neutrons, carbon-14, -15, and -16, each eject an electron.