Relative Atomic Mass
The weighted average mass of the atoms of an element, expressed relative to one twelfth the mass of a carbon-12 atom.
Formula / equation
A_r = Σ(isotope abundance × isotope mass)
Unit
Dimensionless
In depth
Because elements occur as isotopic mixtures, the value on the periodic table is an abundance-weighted average, not the mass of any single atom. This is why chlorine is quoted as 35.45 rather than 35 or 37. Values are revised periodically by IUPAC as isotopic measurements improve.
Examples in the real world
Chlorine is 75.8% ³⁵Cl and 24.2% ³⁷Cl, giving A_r = 0.758 × 35 + 0.242 × 37 ≈ 35.45.