Explain why the mass spectrum of naturally occurring chlorine shows two groups of peaks, and explain how the data from such a spectrum can be used to determine the relative atomic mass of chlorine.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chlorine exists as two naturally occurring isotopes: chlorine-35 and chlorine-37, with approximate natural abundances of 75% and 25% respectively.
Model answer (4 marks)
Chlorine has two naturally occurring isotopes, Cl‑35 and Cl‑37, which differ by two neutrons (35 u and 37 u). In a mass spectrum the ionised atoms are separated by mass‑to‑charge ratio, so the two isotopes give two separate groups of peaks.
The relative heights (or integrated areas) of the peaks in each group are proportional to the natural abundances of the isotopes. Thus the Cl‑35 peaks are about three times higher than the Cl‑37 peaks.
The relative atomic mass of chlorine is the weighted mean of the isotope masses:
Ar(Cl) = (35 u × 0.75) + (37 u × 0.25) = 26.25 u + 9.25 u = 35.5 u.
This value, expressed relative to 1/12 the mass of carbon‑12, gives the accepted relative atomic mass of chlorine.
The relative heights (or integrated areas) of the peaks in each group are proportional to the natural abundances of the isotopes. Thus the Cl‑35 peaks are about three times higher than the Cl‑37 peaks.
The relative atomic mass of chlorine is the weighted mean of the isotope masses:
Ar(Cl) = (35 u × 0.75) + (37 u × 0.25) = 26.25 u + 9.25 u = 35.5 u.
This value, expressed relative to 1/12 the mass of carbon‑12, gives the accepted relative atomic mass of chlorine.
Examiner tips
- Mention the two isotopes and their mass difference first. Show the proportionality between peak height and abundance. Write the weighted‑mean formula clearly. Include the numerical calculation to 35.5 u.
Common mistakes
- Confusing mass‑to‑charge ratio with mass. Using the wrong isotope masses or abundances. Omitting the weighted‑mean calculation or the 1/12 C‑12 reference.
Mark scheme (4 marks)
- Chlorine has two isotopes that differ in the number of neutrons (chlorine-35 has 18 neutrons; chlorine-37 has 20 neutrons), so two different masses of Cl⁺ ions are detected, producing two distinct groups of peaks.
- The relative heights (or areas) of the two peaks reflect the relative natural abundances of the two isotopes.
- The relative atomic mass is calculated as the weighted mean mass of all isotopes relative to 1/12 the mass of carbon-12, using each isotope's mass and its fractional abundance.
- Using the given abundances: Ar(Cl) = (35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5, which matches the accepted value.
Key terms in this question
mass spectrum · relative atomic mass
Related
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