Magnesium has three isotopes: magnesium-24, magnesium-25 and magnesium-26. The relative atomic mass of magnesium is 24.3. Explain why the relative atomic mass of magnesium is not a whole number and describe what is meant by the term isotope.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Magnesium atoms have the same number of protons (12) but differ in the number of neutrons, giving the three isotopes Mg‑24, Mg‑25 and Mg‑26.
The relative atomic mass is calculated as a weighted average of the masses of the naturally occurring isotopes, with the weights being their relative abundances.
Because the three isotopes are present in different proportions, the weighted average of their masses is not a whole number.
Hence the relative atomic mass of magnesium is 24.3, not an integer.
An isotope is an atom of a given element that has the same number of protons but a different number of neutrons, giving it a different mass number.
The relative atomic mass is calculated as a weighted average of the masses of the naturally occurring isotopes, with the weights being their relative abundances.
Because the three isotopes are present in different proportions, the weighted average of their masses is not a whole number.
Hence the relative atomic mass of magnesium is 24.3, not an integer.
An isotope is an atom of a given element that has the same number of protons but a different number of neutrons, giving it a different mass number.
Examiner tips
- Use the definition of isotope – same Z, different N. Show the weighted‑average idea for relative atomic mass. Explain the link between unequal abundances and a non‑whole number. Keep answer concise, 4 points total.
Common mistakes
- Saying all isotopes have the same mass. Forgetting that the average uses abundances. Confusing atomic number with mass number.
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 is an average value that takes account of the abundance of the isotopes
- The three isotopes have different abundances / they are not present in equal amounts
- Because the weighted average of the isotope masses is not a whole number, the relative atomic mass is not a whole number
Key terms in this question
isotope · relative atomic mass
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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