Relative atomic mass stops being a number on the periodic table

Ar is the number printed on the periodic table. Isotopes are mentioned; weighted-average calculations may appear at higher tier.

Ar is defined on the ¹²C = 12 scale and is a weighted mean over the natural isotopic abundances (Structure 1.4.2), and IB requires you to derive it from a mass spectrum (1.2.3) — reading peak positions (m/z) and relative abundances. Mass spectrometry then reappears in organic analysis (Structure 3.2.8) for molecular ion peaks and fragmentation patterns (loss of 15 = CH₃, 29 = C₂H₅ or CHO, 17 = OH, 45 = COOH).

Ar becomes an experimentally determined, non-integer quantity with a physical origin, and mass spectra become a readable data type rather than a picture.

The number in the box is enough for GCSE mole calculations.

Confusing mass number (integer, one isotope) with relative atomic mass (weighted mean, usually non-integer); misreading the x-axis of a mass spectrum as mass rather than mass/charge; not recognising the M+2 peak pattern for chlorine and bromine.