A level Chemistry · Edexcel 9CH0 · Topic 1A

Atoms, isotopes and mass spectrometry

Key points

Mass number = protons + neutrons; atomic number = protons. Isotopes differ only in neutron number.
A mass spectrometer ionises, accelerates and separates ions by mass-to-charge ratio ( $m/z$ ); the detector measures the abundance of each.
Relative atomic mass $A_r = \frac{\sum(\text{isotope mass} \times \text{abundance})}{\sum \text{abundance}}$ .
For diatomic elements such as Cl₂ you see peaks for each combination of isotopes, e.g. ³⁵Cl³⁵Cl, ³⁵Cl³⁷Cl and ³⁷Cl³⁷Cl at $m/z$ 70, 72 and 74.
The peak with the highest $m/z$ for a molecule (the molecular ion, M⁺) gives its relative molecular mass.
Exam tip: give answers to a sensible number of significant figures, and remember $A_r$ has no units.

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An atom is a tiny, dense nucleus of protons and neutrons surrounded by electrons. Isotopes of an element differ only in their neutron count, and a mass spectrometer lets us measure how much of each isotope is present.

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