Britannica Sources used in this answer

Why is carbon-12 used as a standard for atomic weights?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A labeled diagram of the periodic table entry for carbon, showing its atomic number 6, symbol C, electron configuration, atomic weight range, solid physical state, hexagonal crystal structure, and weakly acidic oxide properties. Color coding and icons indicate it is a nonmetal, solid at room temperature, with hexagonal structure and weak acidity. — carbon Carbon and its properties.

Carbon-12 is used as the international standard for atomic weights because it provides a precise and stable reference point for measuring the atomic mass of all other elements.

In 1961, carbon-12 was officially selected to replace oxygen as the standard for this scale. Under this system, the carbon-12 atom is assigned an exact atomic mass of 12 units. Consequently, one atomic mass unit (amu)—also known as the dalton—is defined as exactly one-twelfth of the mass of a single carbon-12 atom. This isotope is the most abundant form of carbon, making it a reliable and consistent benchmark for scientific measurements.123