What is the most abundant isotope of carbon?

The most abundant isotope of carbon is carbon-12. It accounts for about 98.93% of naturally occurring carbon on Earth, while carbon-13 makes up most of the remainder and carbon-14 exists only in trace quantities.

Carbon-12 contains six protons and six neutrons, giving it a mass number of 12. It is stable and is produced in stars through the triple-alpha process, in which three helium nuclei ultimately combine to form a carbon nucleus. Its cosmic abundance reflects both stellar nucleosynthesis and the stability of its nucleus.

Carbon-12 also has an important measurement history. In 1961, it replaced oxygen as the reference standard for comparing atomic masses. One unified atomic mass unit, or dalton, is defined as one-twelfth of the mass of an unbound carbon-12 atom in its ground state. The mole was historically tied to the number of atoms in exactly 12 grams of carbon-12.

Carbon-13 is also stable and is widely used in isotope-ratio studies. Carbon-14, by contrast, is radioactive, has a half-life of roughly 5700 years, and is central to radiocarbon dating. Carbon-11 is short-lived and is used in some medical PET imaging applications.