Carbon-12 (stable, 99%)

Carbon-12 is the stable isotope with six protons and six neutrons. One-twelfth of the mass of a free carbon-12 atom at rest in its ground state defines the unified atomic mass unit.

Its nucleus has spin zero, so ordinary carbon NMR instead commonly observes carbon-13. The integer twelve in the isotope name is a nucleon count; the measured masses of other isotopes need not be exact integers because nuclear and electronic binding affect mass.

Carbon-12 (\(^12C\)) is a stable isotope of carbon, which is one of the fundamental elements in chemistry and biology. It is the most abundant carbon isotope, accounting for about 98.9% of all naturally occurring carbon.