All calculations
Relative atomic mass from isotopes
Atomic structure
AQA 8462
ORIGINAL
Ar = sum(isotope mass x abundance) / sum(abundances)
The abundances are weights: multiply first, add, then divide by the total abundance.
Verified against AQA 8462 (2026 spec)
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
2 marks
Copper is 69.2% Cu-63 and 30.8% Cu-65. Calculate its relative atomic mass.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Taking an unweighted mean of the isotope masses.
- Using percentage abundances but forgetting to divide by 100.
Exam tip: Your answer must lie between the isotope masses and be nearer the more abundant isotope.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.