GCSE · Chemistry · AQA · Spec 8462

Relative atomic mass

Two isotopes can have very different abundances, so why should they count equally in an average?

What you need to know

  • Relative atomic mass is an average that takes account of the abundance of an element's isotopes.
  • A more abundant isotope has more influence on the relative atomic mass than a less abundant isotope.
  • For percentage abundances, multiply each isotope's relative mass by its percentage abundance, add the contributions, then divide by 100.
  • Use the abundances as a sense-check: the average should lie closer to the isotope that is more abundant.

The big picture

Relative atomic mass is an average for an element that takes account of how abundant each isotope is. Each isotope influences the average according to its percentage abundance. To calculate the relative atomic mass, weight each isotope's relative mass by its percentage abundance, add the contributions, then divide by 100. The result should be pulled towards the isotope that is more abundant.

TONIGHT'S REVISION

Relative atomic mass

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

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.

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 influenceRelative 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.

Key points

1Relative atomic mass is an abundance-weighted average.
2Do not automatically give every isotope equal influence.
3Keep each isotope's relative mass paired with its own percentage abundance.
4Add the weighted contributions before dividing by 100.
5Unequal abundances pull the average towards the more abundant isotope.

Worked example

Problem

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

🧠

Memory hook

Think of abundance as voting power: the isotope with more of the votes pulls the average closer to its value.

⚠ Watch out

Taking a simple mean of the isotope values and ignoring their percentage abundances.

✓

Check yourself

Why should an isotope with 80% abundance pull the relative atomic mass closer to its value than one with 20% abundance?

Flashcards

(6)
What is relative atomic mass?
An average value that takes account of the abundance of an element's isotopes.
Why does isotope abundance matter when finding relative atomic mass?
Because a more abundant isotope has more influence on the average.
What do you do with each isotope's relative mass and percentage abundance?
Multiply them together to find that isotope's weighted contribution.
What do you do after adding the weighted contributions when the abundances are percentages?
Divide the total by 100.
Why can a simple mean give the wrong relative atomic mass?
It treats the isotopes as equally abundant even when their percentage abundances are different.
If one isotope is much more abundant than another, where should the relative atomic mass lie?
Closer to the relative mass of the more abundant isotope.

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