Atoms are far too light to weigh in grams, so chemists compare them with one agreed standard: an atom of carbon-12, fixed at exactly 12. Every relative mass in the Data Booklet is a ratio against that atom.
All relative masses are measured against 1/12 of the mass of one atom of carbon-12 (the unified atomic mass unit, u). Relative isotopic mass is the mass of one atom of an isotope on that scale. Relative atomic mass Ar is the weighted mean mass of the atoms of an element. Relative molecular mass Mr adds the Ar values in a molecule; for giant ionic compounds the same sum is called relative formula mass. None of them has units.
Load an element into the mass spectrometer and read off each isotope's m/z value and abundance. Before the display shows it, estimate where the weighted mean will fall: it is always pulled towards the biggest peak.
| Term | What it measures | Example |
|---|---|---|
| relative isotopic mass | mass of one atom of a particular isotope, relative to 1/12 of the mass of a carbon-12 atom | 3717Cl = 37 |
| relative atomic mass, Ar | weighted mean mass of an atom of the element, relative to 1/12 of the mass of a carbon-12 atom | Cl = 35.5 |
| relative molecular mass, Mr | weighted mean mass of a molecule, relative to 1/12 of the mass of a carbon-12 atom | H2O = 18.0 |
| relative formula mass | the same sum for one formula unit of a giant structure, which has no molecules | NaCl = 58.5 |
Multiply each isotope's mass by its abundance, add the products, and divide by the total abundance. If the abundances are percentages the total is 100.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Relative atomic mass is measured relative to:
What is the relative molecular mass of CO2? (Ar: C 12.0, O 16.0)
Sodium chloride is a giant ionic lattice. The sum of Ar values in NaCl is called its:
Boron is 20% 10B and 80% 11B. Its relative atomic mass is:
Add up the Ar values for every atom in the formula, taking care with brackets and water of crystallisation.
A molecule such as Cl2 gives three molecular ion peaks, at m/z 70, 72 and 74, because each atom can be either isotope. With 75% 35Cl and 25% 37Cl the chances are 0.75 × 0.75, 2 × 0.75 × 0.25 and 0.25 × 0.25, a ratio of 9 : 6 : 1. Do not forget the factor of 2 for the mixed molecule, which can form two ways.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Copper consists of 63Cu and 65Cu only, and its Ar is 63.55. What is the percentage abundance of 63Cu?
Chlorine is 75% 35Cl and 25% 37Cl. Which describes the molecular ion peaks in the mass spectrum of Cl2?
What is the relative formula mass of CuSO4·5H2O? (Ar: Cu 63.5, S 32.1, O 16.0, H 1.0)
Which statement is correct?
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