Neon has two main isotopes: neon-20 and neon-22. The relative atomic mass of neon is 20.2. Explain what this tells us about the abundance of neon-20 compared to neon-22, and explain why the relative atomic mass is not a whole number.

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)

Neon‑20 is the more abundant isotope, so the average mass is nearer to 20 than to 22. The relative atomic mass is a weighted average of the masses of the isotopes, weighted by their natural abundances. Because the two isotopes have different masses, the weighted average is not a whole number.

Examiner tips

  • Use the word ‘weighted average’ to show understanding of isotopic abundance.
  • Show that the average lies between 20 and 22 to justify the dominance of neon‑20.
  • Explain that the non‑whole number arises from the averaging process.
  • Use the exact relative atomic mass 20.2 to support your explanation.

Common mistakes

  • Saying the relative atomic mass is a simple sum of the isotope masses.
  • Claiming the average is a whole number because it is close to 20.
  • Not linking the non‑whole number to the weighted average of different isotope masses.

Mark scheme (4 marks)

  1. Neon-20 is more abundant than neon-22
  2. The relative atomic mass is closer to 20 than to 22, which shows neon-20 is the dominant isotope
  3. Relative atomic mass is an average value that takes account of the abundance of the isotopes of the element
  4. Because it is a weighted average of two isotopes with different masses, the result is not a whole number

Key terms in this question

isotope · relative atomic mass · abundance

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