Atomic Mass of Carbon

Carbon-12 was assigned a mass of exactly 12 u in 1961, ending a long-running split in which chemists used an oxygen-16 scale and physicists used a slightly different one. Natural carbon is not pure ¹²C, though: about 1.07 % is ¹³C, which pulls the standard atomic weight up to 12.011 g/mol. From 1971 to 2019 the mole itself was defined as the number of atoms in 12 g of carbon-12, making this element the pivot of all chemical quantity.

The ±0.002 uncertainty is genuine natural variation. Photosynthesis discriminates against the heavier isotope, so plant carbon is isotopically lighter than atmospheric CO₂, and C₃ and C₄ plants discriminate by different amounts. That signature is how food-fraud laboratories detect cane sugar added to honey, and how atmospheric scientists distinguish fossil carbon from biological carbon in the modern CO₂ record. Radiocarbon dating rides on a third isotope, ¹⁴C, present at about one part in 10¹².