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.

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Why Do Atoms Form Ions?

Atoms form ions to achieve a full outer shell of electrons — this is the most stable electron configuration. A full outer shell is the same configuration as the nearest noble gas.

• Metals have few outer electrons (1, 2 or 3) — it is easier to lose these electrons to achieve a full outer shell.
• Non-metals have many outer electrons (5, 6 or 7) — it is easier to gain a few electrons to fill the outer shell.

This drive to achieve a full outer shell (noble gas configuration) is the fundamental reason why ions form.

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Formation of Positive Ions (Cations)

Metals form positive ions by losing their outer shell electrons. The number of electrons lost equals the group number of the metal.

Group 1 Metals — Lose 1 Electron → +1 Ion

Sodium (Na): 2, 8, 1 → Na⁺: 2, 8

• Sodium has 11 protons and 11 electrons (neutral).
• It loses 1 electron from its outer shell.
• Now it has 11 protons (+11) and 10 electrons (−10).
• Overall charge: +1 → Na⁺
• Electron configuration matches neon (2, 8) — a noble gas configuration.

Group 2 Metals — Lose 2 Electrons → +2 Ion

Magnesium (Mg): 2, 8, 2 → Mg²⁺: 2, 8

• Magnesium loses 2 electrons from its outer shell.
• 12 protons (+12) and 10 electrons (−10).
• Overall charge: +2 → Mg²⁺

Group 3 Metals — Lose 3 Electrons → +3 Ion

Aluminium (Al): 2, 8, 3 → Al³⁺: 2, 8

• Aluminium loses 3 electrons.
• 13 protons (+13) and 10 electrons (−10).
• Overall charge: +3 → Al³⁺

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Formation of Negative Ions (Anions)

Non-metals form negative ions by gaining electrons to fill their outer shell. The number of electrons gained = 8 − group number (for Groups 5, 6 and 7).

Group 7 Non-Metals — Gain 1 Electron → −1 Ion

Chlorine (Cl): 2, 8, 7 → Cl⁻: 2, 8, 8

• Chlorine has 17 protons and 17 electrons (neutral).
• It gains 1 electron to fill its outer shell.
• Now it has 17 protons (+17) and 18 electrons (−18).
• Overall charge: −1 → Cl⁻
• Electron configuration matches argon (2, 8, 8).

Group 6 Non-Metals — Gain 2 Electrons → −2 Ion

Oxygen (O): 2, 6 → O²⁻: 2, 8

• Oxygen gains 2 electrons.
• 8 protons (+8) and 10 electrons (−10).
• Overall charge: −2 → O²⁻

Group 5 Non-Metals — Gain 3 Electrons → −3 Ion

Nitrogen (N): 2, 5 → N³⁻: 2, 8

• Nitrogen gains 3 electrons.
• 7 protons (+7) and 10 electrons (−10).
• Overall charge: −3 → N³⁻

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Predicting Ion Charge from the Periodic Table

The charge of an ion can be predicted from the element's group number:

Group	Electrons Lost/Gained	Ion Charge	Examples
1	Loses 1	+1	Li⁺, Na⁺, K⁺
2	Loses 2	+2	Mg²⁺, Ca²⁺, Ba²⁺
3	Loses 3	+3	Al³⁺
5	Gains 3	−3	N³⁻
6	Gains 2	−2	O²⁻, S²⁻
7	Gains 1	−1	F⁻, Cl⁻, Br⁻, I⁻
0	Neither	0	He, Ne, Ar (no ions)

Exam Tip: Group 4 elements (e.g. carbon, silicon) do not typically form ions by gaining or losing electrons at GCSE level. They have 4 outer electrons — losing or gaining 4 would require too much energy. Instead, they share electrons and form covalent bonds.

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Common Ions You Should Know

Positive Ions (Cations)

Ion	Symbol	Charge
Sodium ion	Na⁺	+1
Potassium ion	K⁺	+1
Lithium ion	Li⁺	+1
Magnesium ion	Mg²⁺	+2
Calcium ion	Ca²⁺	+2
Aluminium ion	Al³⁺	+3
Iron(II) ion	Fe²⁺	+2
Iron(III) ion	Fe³⁺	+3
Copper(II) ion	Cu²⁺	+2
Zinc ion	Zn²⁺	+2
Hydrogen ion	H⁺	+1
Ammonium ion	NH₄⁺	+1

Negative Ions (Anions)

Ion	Symbol	Charge
Chloride ion	Cl⁻	−1
Bromide ion	Br⁻	−1
Iodide ion	I⁻	−1
Fluoride ion	F⁻	−1
Oxide ion	O²⁻	−2
Sulfide ion	S²⁻	−2
Hydroxide ion	OH⁻	−1
Nitrate ion	NO₃⁻	−1
Sulfate ion	SO₄²⁻	−2
Carbonate ion	CO₃²⁻	−2

Exam Tip: Transition metals (like iron and copper) can form ions with different charges. This is why we write iron(II) for Fe²⁺ and iron(III) for Fe³⁺. The Roman numeral tells you the charge. You need to know the common ions listed above for the Edexcel exam.

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Ions and Noble Gas Configuration