Silicon has two naturally occurring isotopes: silicon-28 and silicon-29. Explain why the relative atomic mass of silicon 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).

Silicon is used in the manufacture of computer chips. It has an atomic number of 14.

Model answer (4 marks)

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Silicon has two naturally occurring isotopes,

• silicon‑28 (mass number 28) and
• silicon‑29 (mass number 29).

Because the two isotopes have different masses, the relative atomic mass of silicon is calculated as a weighted average of the two masses, with the weights being the natural abundances of each isotope. The abundances are not equal (they are not 50 % each), so the weighted average falls between 28 and 29. Consequently the relative atomic mass of silicon is not a whole number but a value such as 28.09.

Examiner tips

  • Use the term ‘isotopes’ and state the mass numbers of the two isotopes.
  • Explain that the relative atomic mass is a weighted average that depends on natural abundances.
  • Show that unequal abundances give a non‑whole number average.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Failing to mention that the two isotopes have different masses.
  • Confusing atomic number with mass number.
  • Assuming the average is 28.5 because the isotopes are 50:50.

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 two isotopes have different masses / mass numbers (silicon-28 has a lower mass than silicon-29)
  3. Relative atomic mass is an average value that takes account of the abundance of the isotopes
  4. Because the two isotopes exist in proportions that are not equal (not 50:50), the average mass falls between whole numbers, giving a non-whole number value

Key terms in this question

isotope · relative atomic mass

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