Relative atomic and formula mass · Year 9
Atoms are far too light to weigh in grams, so chemists compare their masses with an atom of carbon-12. The relative atomic mass, Aᵣ, of an element is the mean mass of its atoms on this scale. Adding up the Aᵣ of every atom in a formula gives the relative formula mass, Mᵣ, and from that you can work out how much of a compound's mass comes from each element.
Key facts
- Aᵣ is the mean mass of an element's atoms compared with 1/12 of the mass of a carbon-12 atom. It has no units.
- Aᵣ is a weighted mean of the isotopes, so it need not be a whole number: chlorine is 75% chlorine-35 and 25% chlorine-37, so Aᵣ = 35.5.
- Aᵣ = (mass of each isotope × its percentage, added up) ÷ 100.
- Mᵣ is the sum of the Aᵣ of all the atoms in the formula: Mᵣ of H2O = 2 × 1 + 16 = 18.
- A number after a bracket multiplies everything inside it: Ca(OH)2 has 1 Ca, 2 O and 2 H.
- Percentage by mass of an element = (Aᵣ × number of its atoms) ÷ Mᵣ × 100.
- The mass of an element in a sample = mass of sample × (Aᵣ × number of atoms) ÷ Mᵣ.