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 this information about boron to explain why its relative atomic mass is closer to 11 than to 10.

Eduqas GCSE Chemistry — 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, weighted by the natural abundances of its isotopes, and is expressed relative to 1/12 of the mass of a carbon‑12 atom.

Boron has two isotopes, ^10B and ^11B, with a relative atomic mass of 10.8. Because the average is 10.8, the contribution of ^11B must be larger than that of ^10B; otherwise the average would be nearer 10. Therefore ^11B is the more abundant isotope, and its greater abundance pulls the weighted average closer to 11 than to 10.

Examiner tips

  • Define relative atomic mass as a weighted average of isotope masses; mention comparison to C‑12. Explain that a higher abundance of ^11B gives it more weight in the average, moving the value toward 11. Use the given value 10.8 to show the shift.
  • Use the exact terminology: "weighted average", "natural abundance", "relative atomic mass".
  • common_mistakes
  • :

Mark scheme (4 marks)

  1. Relative atomic mass is an average value that takes account of the abundance of the isotopes of the element
  2. Relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom
  3. Boron-11 must be the more abundant isotope (present in greater proportion / higher percentage)
  4. Because boron-11 is more abundant, it has a greater influence on the average, pulling the relative atomic mass closer to 11

Key terms in this question

relative atomic mass · isotope

Related

More Chemical analysis questions

▶ Try answering this question with AI marking (free) →