Explain why atoms of the same element can have different masses, and outline how the relative atomic mass of an element is calculated from its isotopic data.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Atoms of the same element have the same number of protons but can differ in the number of neutrons in the nucleus. These different forms are called isotopes, and the differing neutron number gives each isotope a different mass number.
The relative atomic mass of an element is calculated as the weighted mean of the masses of all naturally occurring isotopes. Each isotopic mass (relative to 1/12 the mass of ‘C’–12) is multiplied by its fractional (or percentage) abundance, and the products are summed. The result is the element’s relative atomic mass.
The relative atomic mass of an element is calculated as the weighted mean of the masses of all naturally occurring isotopes. Each isotopic mass (relative to 1/12 the mass of ‘C’–12) is multiplied by its fractional (or percentage) abundance, and the products are summed. The result is the element’s relative atomic mass.
Examiner tips
- Use the term "isotopes" and "neutrons" to show understanding of the cause of mass differences.
- Show the weighted‑mean formula: Σ(mass × abundance).
- Mention that the scale is relative to 1/12 the mass of carbon‑12.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing atomic number with mass number.
- Using the wrong scale (e.g., absolute mass instead of relative to C-12).
- Failing to mention the weighted mean or the role of isotopic abundance.
Mark scheme (4 marks)
- Atoms of the same element have the same number of protons (atomic/proton number) but can differ in the number of neutrons in the nucleus.
- These different forms of the same element are called isotopes, and the differing neutron number gives each isotope a different mass number (nucleon number).
- Relative atomic mass is calculated as the weighted mean/average of the masses (relative isotopic masses) of all naturally occurring isotopes of the element.
- Each isotopic mass is multiplied by its fractional (or percentage) abundance, and the results are summed; the scale is relative to 1/12 the mass of carbon-12.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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