2.1.1—Unified atomic mass unit

The unified atomic mass unit, u, is defined as one twelfth of the mass of one carbon-12 atom. It provides a common reference scale for comparing atomic and isotopic masses, which are too small to weigh conveniently as individual absolute masses.

Use u as the reference in a ratio: a particle with a relative isotopic mass near 12 is being compared with the one-twelfth carbon-12 standard. The reference scale and the absolute mass of a particular atom are different quantities.

Keep the definition exact: one u is one twelfth of carbon-12, not the whole carbon-12 mass. Relative masses such as Aᵣ and Mᵣ are ratios and have no units; do not attach grams or kilograms to them, and do not confuse them with a nuclide’s mass number.

This card establishes the reference scale only. Isotope-abundance weighting and the separate definitions of relative atomic, molecular and formula mass belong to the neighbouring 2.1 objective; mole and stoichiometric calculations belong to later topics.