Carbon-12

Carbon-12 is a stable isotope of carbon with six protons and six neutrons. Its atomic mass defines the unified atomic mass unit exactly as one-twelfth of a carbon-12 atom’s mass.

Atomic nucleus: The dense central region of an atom, containing protons and neutrons. Carbon-12’s six protons and six neutrons occupy its nucleus.
Nuclear binding energy: The energy required to separate a nucleus into its constituent protons and neutrons. Binding energy accounts for why carbon-12’s mass is less than its separated nucleons’ total mass.
Atomic weight: The relative average mass of an element’s atoms, weighted by their natural isotopic abundances. A carbon-based atomic-weight scale preceded the modern carbon-12 mass standard.
Radiocarbon dating: A method for estimating the age of once-living material from its carbon-14 content. It relies on comparing radioactive carbon-14 with stable carbon isotopes, including carbon-12.
Proton: A positively charged subatomic particle found in atomic nuclei. Six protons give carbon-12 its atomic number and elemental identity.
Strong nuclear force: The fundamental interaction that binds quarks and, at nuclear scales, holds nucleons together. It helps bind carbon-12’s protons and neutrons despite proton-proton repulsion.
Oxygen scale: A historical atomic-mass scale that used oxygen as its reference element. Its competing chemical and physical versions helped motivate a unified carbon-12 standard.
Stable isotope analysis: The measurement and interpretation of stable isotope ratios in samples. Carbon-12 is the reference isotope in commonly reported carbon isotope ratios.
Neutron: An electrically neutral subatomic particle found in most atomic nuclei. Carbon-12 contains six neutrons alongside its six protons.
Nuclear shell model: A model describing nucleons as occupying quantized energy levels within atomic nuclei. It provides one framework for understanding carbon-12’s nuclear structure and stability.

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