What mistakes do students make with Relative atomic mass?

Forgetting to divide by 100 when using percentage abundances. Divide the sum of the products by 100. Using the number of isotopes rather than their abundances as weights. Weight each mass by its percentage or decimal abundance. Adding the isotope mass numbers and dividing by the number of isotopes. Use the abundance-weighted method.

What are the exam tips for Relative atomic mass in Chemistry AQA GCSE?

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.

AQA · GCSE · Chemistry

Course version: 8462 · Version from Sept 2018

Students should be able to calculate the relative atomic mass of an…

Full specification wording

Relative atomic mass: Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes.

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.

Checked against the AQA 8462 specification (opens in a new tab) on 21 September 2026.

Point 2 of 2

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

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.

Common mistakes and how to put them right

Forgetting to divide by 100 when using percentage abundances.
Put it right: Divide the sum of the products by 100.
Using the number of isotopes rather than their abundances as weights.
Put it right: Weight each mass by its percentage or decimal abundance.
Adding the isotope mass numbers and dividing by the number of isotopes.
Put it right: Use the abundance-weighted method.

Exam 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.

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

Boron has two isotopes: ¹⁰B at 20% abundance and ¹¹B at 80% abundance. What is the relative atomic mass of boron?

Tip: press 1 to 4 to choose

Listen

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. 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. A strong answer earns marks for these 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. A common slip: Forgetting to divide by 100 when using percentage abundances. Instead, divide the sum of the products by 100. Exam tip: Lay out the calculation as [ (mass × abundance) + (mass × abundance) ] ÷ 100 to keep it mathematically correct.

See it

$A top-to-bottom flowchart. The first box says check percentage abundances sum to 100%. An arrow leads to a box saying multiply each isotope's mass number by its percentage abundance. An arrow leads to a box saying add the products for all isotopes. An arrow leads to a box saying divide the total by 100. An arrow leads to a final box saying the answer is a weighted average. Separately, a box saying if abundances are decimals or fractions leads by an arrow to a box saying multiply each mass by its decimal abundance and add, without dividing by 100.$

Shows the steps for calculating relative atomic mass from percentage abundances, including the alternative route when abundances are given as decimals or fractions.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

$The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart. The first box says check percentage abundances sum to 100%. An arrow leads to a box saying multiply each isotope's mass number by its percentage abundance. An arrow leads to a box saying add the products for all isotopes. An arrow leads to a box saying divide the total by 100. An arrow leads to a final box saying the answer is a weighted average. Separately, a box saying if abundances are decimals or fractions leads by an arrow to a box saying multiply each mass by its decimal abundance and add, without dividing by 100.$

1Box 1
2Box 2
3Box 3

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