Silicon has three naturally occurring isotopes: silicon-28, silicon-29, and silicon-30. The relative atomic mass of silicon is 28.1. Explain what is meant by the term 'isotope' and use the information given to explain why the relative atomic mass of silicon is not a whole number.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
An isotope is an atom of a given element that has the same number of protons but a different number of neutrons.
The relative atomic mass of an element is calculated as the weighted average of the masses of its naturally occurring isotopes, where the weights are their relative abundances.
For silicon, silicon‑28 is the most abundant isotope, so the average mass is close to 28.
Because the heavier isotopes silicon‑29 and silicon‑30 are also present in smaller amounts, the weighted average is pulled slightly above 28, giving the non‑whole number value of 28.1.
The relative atomic mass of an element is calculated as the weighted average of the masses of its naturally occurring isotopes, where the weights are their relative abundances.
For silicon, silicon‑28 is the most abundant isotope, so the average mass is close to 28.
Because the heavier isotopes silicon‑29 and silicon‑30 are also present in smaller amounts, the weighted average is pulled slightly above 28, giving the non‑whole number value of 28.1.
Examiner tips
- Define ‘isotope’ and explain the concept of weighted average; mention abundance; keep answer concise and use correct terminology.
- Show the logical link: most abundant isotope → average near 28, heavier isotopes → average >28.
- Use the exact figures (28.1) to demonstrate understanding of the calculation.
Common mistakes
- Confusing ‘isotope’ with ‘isomer’ or ‘isotopic composition’; not recognising that isotopes differ only in neutrons.
- Failing to explain that the average is weighted by abundance, or stating that the mass is simply the sum of all isotope masses.
- Using vague language such as ‘different atoms’ instead of specifying protons and neutrons.
Mark scheme (4 marks)
- Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
- Relative atomic mass takes account of the abundance (proportion) of each isotope
- Silicon-28 is the most abundant isotope, so the relative atomic mass is closest to 28
- Because the heavier isotopes (silicon-29 and silicon-30) are also present, the average is pulled above 28, giving a non-whole number value of 28.1
Key terms in this question
isotope · relative atomic mass
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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