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Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Their chemistry is very similar, while their masses and nuclear stability can differ. One element can therefore have several nuclear versions without changing its place in the periodic table. Radioactive decay is a spontaneous nuclear transformation. For a large population of identical unstable nuclei, the number remaining follows an exponential law. The decay of an individual nucleus is unpredictable, yet a large population obeys a remarkably precise statistical law.
Primary references
IUPAC Gold Book · NIST · BIPM