2.1.2—Atomic mass, Ar, isotopic mass

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.