Boron has two naturally occurring isotopes: boron-10 and boron-11. The relative atomic mass of boron is 10.8. Explain what is meant by the term 'relative atomic mass' and use the value of 10.8 to explain what this tells us about the relative abundance of the two isotopes of boron.

WJEC GCSE Chemistry (Wales) — 2.4 Chemical analysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Relative atomic mass is the average mass of an atom of an element, expressed in atomic mass units and compared to 1/12th the mass of a carbon‑12 atom. The value of 10.8 takes into account the abundance of each isotope. Because 10.8 is closer to 11 than to 10, boron‑11 is the more abundant isotope.

Examiner tips

  • Define relative atomic mass and its reference to C‑12; explain averaging over isotopes; link the numerical value to isotope abundance; keep answer concise and use correct terminology.

Common mistakes

  • Confusing relative atomic mass with atomic mass unit; stating that 10.8 is the mass of a single atom; ignoring the role of isotope abundance; not recognising that 10.8 is nearer to 11 than to 10.

Mark scheme (4 marks)

  1. Relative atomic mass is an average mass of an atom of an element
  2. Compared to 1/12th the mass of a carbon-12 atom
  3. The value takes into account the abundance of each isotope
  4. Because 10.8 is closer to 11 than to 10, boron-11 is the more abundant isotope

Key terms in this question

relative atomic mass · isotope · abundance

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