STUDENT SPACE · TOPIC 3

Isotopes and relative atomic mass

Explain isotope chemistry and calculate a weighted relative atomic mass.

The essentials

Neutral isotopes have the same electronic configuration and therefore the same chemical properties.
Relative atomic mass is an abundance-weighted average compared with one twelfth of the mass of carbon-12. It has no unit.
For percentage abundances, multiply each isotope mass by its abundance, add the results and divide by 100.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

A sample contains 50% isotope-20 and 50% isotope-22. What is its relative atomic mass?

Need a hint for Question 1?

Both isotopes contribute equally to the average.

Check Question 1

(20 × 50 + 22 × 50) ÷ 100 = 21. This does not mean each atom has mass number 21.

Calculate Aᵣ for a sample with 60% isotope-63 and 40% isotope-65. Show your working.

Need a hint for Question 2?

Multiply each isotope mass by its fractional abundance.

Check Question 2

Aᵣ = (63 × 60 + 65 × 40) ÷ 100 = 63.8.

A sample has isotope masses 24, 25 and 26 with abundances 80%, 10% and 10%. Calculate Aᵣ and explain why it is closer to 24.

Need a hint for Question 3?

The most abundant isotope contributes most to the mean.

Check Question 3

Aᵣ = (24 × 80 + 25 × 10 + 26 × 10) ÷ 100 = 24.3. Isotope-24 is much more abundant.

Use the matching chapter worksheet. Attempt independently, compare your reasoning with your partner, then bring unresolved questions to your teacher.

Change the number of neutrons and compare isotope mixtures.