Ions and Ion Formation

This lesson covers how ions are formed when atoms gain or lose electrons — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand why atoms form ions, how to predict the charge of an ion from its position in the periodic table, and how to write the formulae of common ions.

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What Is an Ion?

An ion is a charged particle formed when an atom gains or loses one or more electrons. Because the number of protons stays the same but the number of electrons changes, the atom is no longer electrically neutral — it has an overall positive or negative charge.

Two Types of Ions

Type of Ion	Formed By	Charge	Typical Elements
Cation (positive ion)	Atom loses electrons	Positive (+)	Metals
Anion (negative ion)	Atom gains electrons	Negative (−)	Non-metals

Exam Tip: Remember: metals form positive ions (cations) by losing electrons; non-metals form negative ions (anions) by gaining electrons. A useful mnemonic: "Metals Lose Positively" — metals lose electrons to form positive ions.