By Basil Phillips – Updated Mar 24, 2022
The nucleus of each atom contains protons, neutrons, and electrons. While the number of protons and electrons is constant for a given element, the number of neutrons can vary, giving rise to different isotopes. Carbon, like many other elements, has one isotope that is overwhelmingly more common than the others.
Carbon‑12 is the most abundant carbon isotope, accounting for nearly 99 % of naturally occurring carbon on Earth. Its nucleus comprises six protons and six neutrons, yielding a total mass of exactly 12.000 atomic mass units (amu). Because of this exact mass, carbon‑12 serves as the international reference standard for all other atomic masses.
Besides carbon‑12, two other stable isotopes occur naturally. Carbon‑13, which contains seven neutrons, represents about 1 % of terrestrial carbon. Carbon‑14, with eight neutrons, is radioactive and present at a level of roughly two‑trillionths of natural carbon. Scientists have also synthesized short‑lived isotopes ranging from carbon‑8 to carbon‑22, but these unstable forms have limited practical applications.
Because living organisms preferentially incorporate carbon‑12, the ratio of carbon‑13 to carbon‑12 in biological materials is slightly lower than the environmental ratio. By measuring this ratio in ice cores, tree rings, and marine sediments, scientists can infer past atmospheric carbon dioxide concentrations and oceanic uptake rates, providing critical insights into historical climate change.
Carbon‑14 is unique among carbon isotopes in that it is radioactive. It decays with a half‑life of 5,730 years, releasing beta radiation. All living organisms absorb carbon‑14 through photosynthesis and respiration; when an organism dies, the remaining carbon‑14 decays at a predictable rate. By measuring the residual carbon‑14 in ancient samples, researchers can determine the age of archaeological and geological materials with remarkable precision—a method known as radiocarbon dating.
