Relative atomic mass — AQA GCSE Chemistry
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Relative atomic mass explained
To calculate relative atomic mass from percentage abundances, multiply each isotope's mass number by its percentage abundance, add the products, then divide by 100.
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For example, if an element is 20% isotope-10 and 80% isotope-11, the calculation is ((10 × 20) + (11 × 80)) ÷ 100 = (200 + 880) ÷ 100 = 10.8. The answer is a weighted average and may be quoted to a suitable number of decimal places. If abundances are given as decimals or fractions, multiply each mass by its decimal abundance and add, without dividing by 100. Always check that the abundances total 100% before starting.
Your focus
- Calculate relative atomic mass from percentage abundances of isotopes.
- Adapt the calculation when abundances are given as decimals or fractions.
- Check that a calculated relative atomic mass lies between the isotope mass numbers.
Relative atomic mass exam tips
Marking Points
- Multiply each isotope's mass number by its percentage abundance.
- Add the products for all isotopes.
- Divide the total by 100 when abundances are percentages.
- If abundances are given as decimals, multiply each mass by its decimal abundance and add without dividing by 100.
- Check that percentage abundances sum to 100 before calculating.
- Quote the final answer as a weighted average, often to one decimal place.
Examiner Tips
- 💡Lay out the calculation as [ (mass × abundance) + (mass × abundance) ] ÷ 100 to keep it mathematically correct.
- 💡Check that the abundances add to 100% before you begin.
- 💡Sanity-check the answer: it must lie between the lowest and highest isotope mass numbers.
Common Mistakes
- Forgetting to divide by 100 when using percentage abundances; instead divide the sum of the products by 100.
- Using the number of isotopes rather than their abundances as weights; instead weight each mass by its percentage or decimal abundance.
- Adding the isotope mass numbers and dividing by the number of isotopes; instead use the abundance-weighted method.