Boron has two isotopes, boron-10 and boron-11. The relative atomic mass of boron is 10.8. Explain what the term isotope means and explain why the relative atomic mass of boron is not a whole number.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
Boron has two isotopes, boron‑10 and boron‑11, which differ in their neutron numbers.
The relative atomic mass of an element is an average of the masses of its naturally occurring isotopes, weighted by their relative abundances.
Because boron‑11 is more abundant than boron‑10, the weighted average is slightly above 10, giving the relative atomic mass of 10.8, which is not a whole number.
Boron has two isotopes, boron‑10 and boron‑11, which differ in their neutron numbers.
The relative atomic mass of an element is an average of the masses of its naturally occurring isotopes, weighted by their relative abundances.
Because boron‑11 is more abundant than boron‑10, the weighted average is slightly above 10, giving the relative atomic mass of 10.8, which is not a whole number.
Examiner tips
- Define isotopes clearly – same protons, different neutrons. Explain the weighted average and mention the different abundances. Show the link between the heavier isotope being more common and the non‑whole number result.
- common_mistakes
- :
- Saying isotopes have different numbers of protons. Forgetting to mention the relative abundances. Claiming the average is a simple arithmetic mean without weighting.
Mark scheme (4 marks)
- Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
- Boron-10 and boron-11 have different abundances (they are not present in equal amounts)
- Relative atomic mass is an average value that takes account of the abundance of the isotopes
- Because the average is weighted towards boron-11 (the more abundant isotope), the result is not a whole number
Key terms in this question
isotope · relative atomic mass
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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