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 information given to explain why the relative atomic mass of boron is closer to 11 than to 10.

Eduqas A-Level Chemistry — 2.3 Transition metals (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of an atom of carbon‑12.

It takes account of the abundance of each isotope.

Because boron‑11 is more abundant than boron‑10, the weighted average is pulled closer to 11 than to 10, giving a relative atomic mass of 10.8.

Examiner tips

  • Define relative atomic mass in terms of C‑12; mention isotope abundances; explain weighted average; keep answer concise to fit 5 marks.

Common mistakes

  • Confusing relative atomic mass with atomic mass unit; ignoring isotope abundance; giving only the definition without explanation of why 10.8 is nearer 11.

Mark scheme (5 marks)

  1. Relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12
  2. Relative atomic mass takes account of the abundance (proportion) of each isotope
  3. Boron-11 must be more abundant than boron-10
  4. Because boron-11 is more abundant, the average is pulled closer to 11 than to 10

Key terms in this question

relative atomic mass · isotope

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