Isotopes and Relative Atomic Mass

This lesson covers isotopes and how to calculate relative atomic mass — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand what isotopes are, why they have the same chemical properties, and be able to calculate relative atomic mass from isotopic abundance data.

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What Are Isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.

Key Points About Isotopes

• Isotopes have the same atomic number (same number of protons).
• Isotopes have different mass numbers (different numbers of neutrons).
• Isotopes have the same electron configuration (because the number of electrons equals the number of protons in a neutral atom).
• Isotopes have the same chemical properties because chemical reactions depend on electron configuration, which is the same for all isotopes of an element.
• Isotopes have different physical properties (e.g. different mass, different density, different rate of diffusion).