# 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.

> IB DP Chemistry Higher Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks

> Chlorine exists as two naturally occurring isotopes: chlorine-35 and chlorine-37, with approximate natural abundances of 75% and 25% respectively.

## Mark scheme (4 marks)

1. 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.
2. The relative heights (or areas) of the two peaks reflect the relative natural abundances of the two isotopes.
3. 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.
4. 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

- [mass spectrum](https://www.gradenine.co.uk/glossary/mass-spectrum)
- [relative atomic mass](https://www.gradenine.co.uk/glossary/relative-atomic-mass)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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