Chlorine has two main isotopes: chlorine-35 and chlorine-37. Explain what is meant by the term isotope and describe how the existence of isotopes explains why the relative atomic mass of chlorine is not a whole number.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The relative atomic mass of chlorine is 35.5.
Model answer (5 marks)
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
Chlorine‑35 and chlorine‑37 are the two main isotopes of chlorine; they have mass numbers 35 and 37 respectively.
The relative atomic mass is calculated as a weighted average of the masses of all naturally occurring isotopes, taking their relative abundances into account.
Because chlorine‑35 is more abundant than chlorine‑37, the weighted average is pulled closer to 35.
Thus the average mass (35.5) is not a whole number, reflecting the contribution of the two isotopes.
Chlorine‑35 and chlorine‑37 are the two main isotopes of chlorine; they have mass numbers 35 and 37 respectively.
The relative atomic mass is calculated as a weighted average of the masses of all naturally occurring isotopes, taking their relative abundances into account.
Because chlorine‑35 is more abundant than chlorine‑37, the weighted average is pulled closer to 35.
Thus the average mass (35.5) is not a whole number, reflecting the contribution of the two isotopes.
Examiner tips
- Define isotopes and give the mass numbers of the two main isotopes. Explain that the relative atomic mass is a weighted average. Show how the greater abundance of chlorine‑35 pulls the average toward 35.
- common_mistakes
- :
- Saying isotopes differ in the number of protons. Forgetting to mention the weighted average or the relative abundances. Giving the wrong mass numbers for the isotopes.
Mark scheme (5 marks)
- Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
- Chlorine-35 and chlorine-37 have different mass numbers (35 and 37 respectively).
- 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.
- Because relative atomic mass is a weighted average of the masses of all isotopes, the value (35.5) is not a whole number.
Key terms in this question
isotope · relative atomic mass
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