Ions, Isotopes and Relative Atomic Mass

This lesson covers the formation of ions, the definition of isotopes, and the calculation of relative atomic mass, as required by the Edexcel GCSE Combined Science specification (1SC0). These are fundamental concepts that underpin much of chemistry at GCSE level.

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What Is an Ion?

An ion is an atom (or group of atoms) that has gained or lost electrons and therefore has an overall electrical charge.

• Atoms are electrically neutral because they have equal numbers of protons and electrons.
• When an atom loses electrons, it becomes a positive ion (called a cation) because there are now more protons than electrons.
• When an atom gains electrons, it becomes a negative ion (called an anion) because there are now more electrons than protons.

Why Do Atoms Form Ions?

Atoms form ions to achieve a full outer electron shell — the same stable electron configuration as a noble gas. This is the most energetically stable arrangement.

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Formation of Positive Ions (Cations)

Metal atoms form positive ions by losing electrons from their outer shell.