Primordial nuclide

A nuclide present on Earth since its formation, either stable or radioactive with a sufficiently long half-life to persist naturally.

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Radioactive decay: The spontaneous transformation of an unstable atomic nucleus into a different nucleus, often with emitted particles or radiation. Decay progressively depletes radioactive primordial nuclides according to their half-lives.
Nuclide: An atomic species defined by its numbers of protons and neutrons in the nucleus. Primordial status applies to a specific nuclear species, not just an element.
Radiometric dating: A method for determining age from measured proportions of radioactive parent nuclides and their decay products. Primordial radioactive nuclides and their products date rocks and minerals.
Uranium-238: A radioactive uranium isotope with a half-life of about 4.5 billion years. Its half-life is comparable to Earth’s age, allowing substantial primordial uranium-238 to remain.
Cosmogenic nuclide: A nuclide formed by cosmic-ray interactions with atoms in the atmosphere or at Earth’s surface. Unlike primordial nuclides, cosmogenic nuclides are continually produced after Earth formed.
Half-life: The time required for half the radioactive nuclei in a sample to decay. Half-life determines whether a radioactive nuclide can persist since Earth formed.
Isotope: One of two or more nuclides of the same element with different numbers of neutrons. Primordial nuclides include individual isotopes, which can differ greatly in stability.
Uranium–lead dating: A radiometric dating method using decay of uranium isotopes to lead isotopes. Long-lived primordial uranium isotopes make this method effective for ancient rocks.
Uranium-235: A radioactive uranium isotope with a half-life of about 704 million years. It survives naturally from Earth’s formation, though far less remains than uranium-238.
Radiogenic nuclide: A nuclide formed by radioactive decay of a different nuclide. Radiogenic nuclides are decay products rather than survivors inherited from Earth’s formation.