12c

Carbon-12 (12C) is a stable isotope of carbon with atomic number 6 and mass number 12. Its nucleus contains six protons and six neutrons. By definition, 12C has an atomic mass of exactly 12 atomic mass units (u). One mole of 12C contains Avogadro’s number of atoms and has a mass of exactly 12 grams, establishing the relationship between the atomic mass unit, the mole, and the gram.

In nature, 12C is by far the most abundant carbon isotope, making up about 98.9% of terrestrial carbon; the remainder consists mainly of 13C (≈1.1%) and trace amounts of other isotopes. 12C is stable and has nuclear spin I = 0, rendering it non-radioactive.

12C plays a central role in defining mass and chemical analysis. The atomic mass unit is defined relative to 12C, and the mole is tied to Avogadro’s number; together, they ensure that the molar mass of carbon is exactly 12 g/mol. 12C is commonly used as a calibration standard in mass spectrometry and as a baseline in isotopic abundance measurements.

In astronomy and nuclear physics, 12C is produced in stars via the triple-alpha process, where three helium nuclei combine to form carbon-12, facilitated by the Hoyle state. This process is essential for the cosmic abundance of carbon and organic chemistry in the universe.

Applications include isotopic labeling and studies of metabolic pathways, and in geology, archaeology, and biology to analyze environmental and biological processes through 12C/13C ratios.