Explain why the relative atomic mass of chlorine is 35.5, rather than a whole number.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Chlorine-35 is more abundant than chlorine-37.
Model answer (4 marks)
Isotopes are atoms of the same element that have the same number of protons but different numbers 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.
Chlorine‑35 (mass ≈ 35 u) is more abundant than chlorine‑37 (mass ≈ 37 u). Because the heavier isotope is less common, the weighted average is pulled closer to 35 u.
The weighted average of the two masses gives a value of about 35.5 u, which is therefore the relative atomic mass of chlorine, not a whole number.
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.
Chlorine‑35 (mass ≈ 35 u) is more abundant than chlorine‑37 (mass ≈ 37 u). Because the heavier isotope is less common, the weighted average is pulled closer to 35 u.
The weighted average of the two masses gives a value of about 35.5 u, which is therefore the relative atomic mass of chlorine, not a whole number.
Examiner tips
- Use the definition of isotopes and weighted average; show the calculation conceptually; mention the higher abundance of Cl‑35; keep answer concise and use correct terminology.
- Include the key point that the average is non‑whole because of different masses and abundances.
Common mistakes
- Saying the atomic mass is 35 because of the most abundant isotope; ignoring the contribution of Cl‑37.
- Using the wrong definition of relative atomic mass (e.g., not a weighted average).
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 of each isotope
- Chlorine-35 is more abundant than chlorine-37, so the average is pulled closer to 35
- The result is a non-whole number (35.5) because it is an average of two isotopes with different masses
Key terms in this question
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