Nuclide

A nuclide is a species of atom specified by its numbers of protons and neutrons, sometimes including a particular nuclear energy state.

Atomic number: The number of protons in an atomic nucleus, which determines the chemical element. Nuclides with the same atomic number belong to the same element.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus, usually with emission of particles or radiation. Decay changes a nuclide into another nuclear species.
Radiometric dating: A method for determining age by measuring radioactive decay in a material. Known decay rates of particular nuclides provide clocks for rocks and artifacts.
Element: A substance whose atoms all have the same number of protons. An element groups multiple nuclides that share one proton number.
Frederick Soddy: A British chemist who established the concept of isotopes and received the 1921 Nobel Prize in Chemistry. His isotope work helped establish the distinctions among nuclear species.
Neutron number: The number of neutrons in an atomic nucleus. Together with proton number, it distinguishes nuclides of one element.
Nuclear stability: The tendency of an atomic nucleus to remain intact rather than undergo radioactive decay. Stability depends on the proton–neutron combination that defines a nuclide.
Radiotracer: A radioactive substance used to track the movement or distribution of atoms in a system. A nuclide's detectable emissions can reveal where its atoms travel.
Isobar: One of multiple nuclides with the same mass number but different atomic numbers. Isobars share total nucleon count, unlike isotopes, which share proton count.
Margaret Todd: A Scottish physician who proposed the term “isotope” at Frederick Soddy's suggestion. The naming of isotopes supplied a key category of nuclide.

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