What is the most common isotope of carbon?

The most common isotope of carbon is Carbon-12. Its nucleus contains six protons and six neutrons, giving it a mass number of 12, and it makes up about 98.93% of naturally occurring carbon on Earth.

Carbon-12 is stable, as is the much less abundant Carbon-13. Carbon-14 is radioactive and is famous for radiocarbon dating, but it occurs only in trace quantities. Carbon-11 is also radioactive, with a short half-life, and is used in some medical imaging applications rather than being the dominant natural form.

Carbon-12 has an important scientific history. In 1961, it replaced oxygen as the reference standard for comparing atomic masses. The mole was historically defined using the number of atoms in 12 grams of Carbon-12, linking this isotope to chemistry’s measurement system. The modern SI definition of the mole was revised in 2019, but Carbon-12 remains the reference for the atomic-mass scale.

Most Carbon-12 is formed in stars through the triple-alpha process, in which helium nuclei ultimately combine to create carbon. That stellar origin helps explain why this isotope is so abundant throughout the cosmos.