2.1 Relative masses of atoms and molecules

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.

Relative atomic mass, Aᵣ, is the weighted average mass of the atoms of an element compared with one twelfth of the mass of one carbon-12 atom. It is a ratio and therefore has no units; the isotope abundances of the element determine the weighting.

For percentage abundances, calculate each isotope contribution, add the contributions, then divide by 100: Aᵣ = Σ(isotopic mass × percentage abundance) / 100. If fractional abundances are supplied instead, multiply by those fractions and add. The result need not be an integer.

Keep the objects distinct: relative isotopic mass describes one specified isotope; relative molecular mass is found by adding the relative atomic masses in one molecule; relative formula mass uses the simplest formula unit for an ionic compound and is calculated in the same additive way.

Do not use the mass number of the most abundant isotope as the element’s Aᵣ, attach units to Aᵣ or Mᵣ, or treat an ionic lattice as a collection of discrete molecules. Use the stated isotope abundances and formula, and leave mole/stoichiometric calculations to later topics.