Atomic Structure Problem Solving

This lesson brings together everything from the previous nine lessons and focuses on the types of multi-step problems you will encounter in Edexcel A-Level exams. We will work through exam-style calculations, data interpretation, and questions that require you to link different concepts together.

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Type 1: Relative Atomic Mass from Mass Spectra

These questions give you isotopic data (either as percentages or as peak heights from a mass spectrum) and ask you to calculate Ar.

Strategy

1. Identify each isotope's mass (m/z value) and abundance
2. If abundances are not percentages, calculate total abundance
3. Apply: Ar = Σ(mass × abundance) / Σ(abundance)

Worked Example

A mass spectrum of rubidium shows two peaks:

m/z	Relative Abundance
85	72.2
87	27.8

Ar = (85 × 72.2 + 87 × 27.8) / (72.2 + 27.8) Ar = (6137.0 + 2418.6) / 100.0 Ar = 8555.6 / 100.0 Ar = 85.6

Reverse Calculation

Sometimes you are given the Ar and asked to find the abundance of one isotope.

Example: Bromine has two isotopes, ⁷⁹Br and ⁸¹Br, with Ar = 79.9. Calculate the percentage abundance of each isotope.