When one of the two neutrons in the tritium nucleus breaks down into a proton and an electron, then a oneproton, two-neutron nucleus of hydrogen is converted into a two-proton, one-neutron nucleus of helium:

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This reaction is illustrated above.

One element is changed into another, and the electron is released from the nucleus as beta radiation. We shall not be concerned with radioactive decay and unstable isotopes, but it is worth noting at least that atomic nuclei are stable when their ratios of neutrons to protons lie within a certain range, namely, 1:1 or with a slight excess of neutrons. With too many neutrons or too many protons, a nucleus becomes unstable and decays spontaneously into a more stable isotope of an element with an atomic number close to that of the original element.