The nuclear structure of a nuclide is classically defined by its number of protons which defines the element, and the number of neutrons which defines the isotope of that element. The sum of protons and neutrons gives the atomic weight. Unstable isotopes or radioactive nuclides will decay with time, while stable isotopes do not. For example, hydrogen has three isotopes ( H to ~3H), although only ~1H and ~3H are stable. H is radioactive with a half-life of 12.4 years. Carbon has 11 isotopes (~8C to ~(18)C) although only ~(12)C and ~(13)C are stable. The others are radioactive with half-lives varying from 5730 years (~(14)C) to less than a millisecond.
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