Magnesium has three naturally occurring isotopes: magnesium-24, magnesium-25, and magnesium-26. The relative atomic mass of magnesium is 24.3. Explain what is meant by the term 'isotope' and explain why the relative atomic mass of magnesium is not a whole number.

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)

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
The three naturally occurring isotopes of magnesium (24, 25 and 26) are present in different proportions.
The relative atomic mass of magnesium is the weighted average of the masses of these isotopes, the weights being their natural abundances.
Because the average is calculated from fractions of the isotopes, the result is not a whole number (24.3).

Examiner tips

  • Define 'isotope' clearly; mention same protons, different neutrons. Explain that natural abundances differ. State that the atomic mass is a weighted average of isotope masses. Show that the weighted average gives a non‑whole number.

Common mistakes

  • Confusing isotope with element. Forgetting to mention different abundances. Saying the atomic mass is a simple sum instead of a weighted average.

Mark scheme (4 marks)

  1. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
  2. The three isotopes of magnesium exist in different amounts / different abundances.
  3. The relative atomic mass is an average value that takes account of the abundance of the isotopes of the element.
  4. Because the average is weighted by abundance, the result is not a whole number.

Key terms in this question

isotope · relative atomic mass

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