GCSE · Chemistry · AQA · Spec 8462

Relative atomic mass

Predict before calculating

Use abundance to decide which isotope should influence the average most.

Practice data: isotope values 10 and 12 occur at 80% and 20% abundance. Which relative atomic mass is sensible?

Build the weighted average

Treat each percentage abundance as that isotope's share of the average.

Give each its weightMultiply each isotope's relative mass by its percentage abundance so the more abundant isotope contributes more.
Combine the contributionsAdd the weighted contributions to bring the effects of all the isotopes into one total.
Return to an averageDivide the total by 100 because the abundances were given as percentages.

The tempting shortcut

Simple meanvsAbundance-weighted average

Compare a simple mean with the abundance-weighted average required here.

Focus

What gets equal influence?

Simple mean

Every isotope value

Abundance-weighted average

Only isotopes with equal abundance

The insight

A simple mean assumes equal influence, while relative atomic mass must reflect the actual percentage abundances.

Practice data: 10 and 12 at 80% and 20%

Simple mean

11.0

Abundance-weighted average

10.4

What information matters?

Simple mean

The isotope values only

Abundance-weighted average

The isotope values and their abundances

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

	Greater influence	Relative atomic mass
50% : 50%Relative masses 10 and 12
80% : 20%Relative masses 10 and 12
20% : 80%Relative masses 10 and 12

Each cell hides a short answer and the reason behind it. Predict before you tap.

Work one through

Problem

Practice data: an element has isotopes with relative masses 24 and 26 at abundances of 75% and 25%. Find its relative atomic mass.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Use isotope abundance to build an average that reflects how much of each isotope is present.

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