Why a relative scale?
A single atom is far too light to weigh directly, so chemists compare masses instead of measuring them in grams. The agreed standard is the carbon-12 isotope. One atom of 12C is assigned a mass of exactly 12 units, and every other atom is compared to one-twelfth of it.
The relative atomic mass (Ar) is the average mass of one atom of an element compared with 1/12 of the mass of one 12C atom. Because it is a ratio, Ar has no units. The value is an average that takes isotope abundance into account, which is why chlorine has Ar = 35.5.
Key idea / Key idea
Ar = (average mass of one atom) ÷ (1/12 × mass of one 12C atom)
Relative molecular mass
The relative molecular mass (Mr) is the sum of the relative atomic masses of every atom shown in the molecular formula. For substances made of ions (like NaCl) we use the same method but call it the relative formula mass.
Worked example
Find Mr of sulfuric acid, H2SO4. Given Ar: H = 1, S = 32, O = 16.
Mr = (2 × 1) + 32 + (4 × 16) = 2 + 32 + 64 = 98.
For water H2O: (2 × 1) + 16 = 18. For carbon dioxide CO2: 12 + (2 × 16) = 44.
Using data carefully
Always read the Ar values given in the question, count the atoms of each element correctly (watch subscripts and brackets), then add. For hydrated salts such as CuSO4·5H2O, include the water of crystallisation: 160 + (5 × 18) = 250.
Remember
- Ar and Mr have no units.
- The scale is based on 12C = 12 exactly.
- Mr = total of all Ar in the formula, including water of crystallisation.