AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Relative atomic mass: Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes.
Relative atomic mass (Ar) is a weighted mean of the masses of an element's isotopes, taking account of how common each isotope is.
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Relative atomic mass (Ar) is a weighted mean of the masses of an element's isotopes, taking account of how common each isotope is. To calculate it, multiply each isotope's mass number by its percentage abundance, add these products, then divide by 100. For example, chlorine is 75% chlorine-35 and 25% chlorine-37, so Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5. The answer is not a whole number because it is an average, and it lies between the isotope mass numbers, closer to the more abundant isotope. Percentage abundances must total 100%; if they do not, check the data before calculating.
What earns the marks
- State that relative atomic mass is a weighted mean of isotope masses, not a simple unweighted average.
- Multiply each isotope's mass number by its percentage abundance.
- Add the products and divide the total by 100 when abundances are percentages.
- Check that percentage abundances sum to 100% and that the final answer lies between the lowest and highest isotope mass numbers.
- Give the answer to an appropriate number of significant figures, usually the same as the data provided.
Common mistakes and how to put them right
Averaging the mass numbers without weighting.
Put it right: Weight each mass number by its percentage abundance before dividing by 100.
Dividing by the number of isotopes instead of 100.
Put it right: When abundances are percentages, the divisor is 100.
Forgetting to convert a decimal abundance to a percentage, or mixing fractions and percentages.
Put it right: Use one consistent form, such as percentages throughout.
Exam tips
- Write the calculation in one line before evaluating it, so the weighting is visible to the examiner.
- Estimate first: the answer must lie between the isotope mass numbers and nearer the more abundant isotope.
- If an answer looks like a whole number, check whether the data were already weighted or whether you divided by the wrong value.
Try the lesson
The start of the Relative atomic mass: Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes. lesson, made from checked teaching notes for this specification point.
Have a go first
A sample of chlorine contains 75% of chlorine-35 atoms and 25% of chlorine-37 atoms. Calculate the relative atomic mass of chlorine in this sample.
Listen
Relative atomic mass (Ar) is a weighted mean of the masses of an element's isotopes, taking account of how common each isotope is. To calculate it, multiply each isotope's mass number by its percentage abundance, add these products, then divide by 100. For example, chlorine is 75% chlorine-35 and 25% chlorine-37, so Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5. The answer is not a whole number because it is an average, and it lies between the isotope mass numbers, closer to the more abundant isotope. Percentage abundances must total 100%; if they do not, check the data before calculating. A strong answer earns marks for these points. State that relative atomic mass is a weighted mean of isotope masses, not a simple unweighted average. Multiply each isotope's mass number by its percentage abundance. Add the products and divide the total by 100 when abundances are percentages. Check that percentage abundances sum to 100% and that the final answer lies between the lowest and highest isotope mass numbers. A common slip: Averaging the mass numbers without weighting: (35 + 37) ÷ 2 = 36. Instead, weight each mass number by its percentage abundance before dividing by 100. Exam tip: Write the calculation in one line before evaluating it, so the weighting is visible to the examiner.
See it
Key words
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Relative atomic mass
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Percentage abundance
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See every definition
- Relative atomic mass
- A weighted mean of the masses of an element's isotopes, taking account of how common each isotope is.
- Percentage abundance
- How common each isotope is, given as a percentage; these must total 100%, and the data should be checked before calculating if they do not.
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