IGCSE Chemistry Ionic Bonding Notes
Ionic bonding is the transfer of electrons from a metal to a non-metal. The resulting ions attract each other in a giant lattice.
Key concepts
- Cations and anions
- Metals lose electrons to form positive cations; non-metals gain electrons to form negative anions.
- Full outer shells
- Ions form so each has the electron configuration of the nearest noble gas.
- Giant ionic lattice
- Repeating 3D structure of alternating positive and negative ions held by electrostatic forces.
- Group 1 → 1+; Group 2 → 2+; Group 3 → 3+; Group 5 → 3−; Group 6 → 2−; Group 7 → 1−.
- Sodium chloride: Na loses 1e⁻ → Na⁺; Cl gains 1e⁻ → Cl⁻.
- Ionic compounds are usually soluble in water and conduct when molten or aqueous.
Worked example
Draw a dot-and-cross diagram for magnesium chloride (MgCl₂).
- 1Mg has 2 outer electrons; each Cl has 7 outer electrons.
- 2Mg transfers one electron to each of two Cl atoms.
- 3Mg becomes Mg²⁺ (empty outer shell shown); each Cl becomes Cl⁻ with 8 outer electrons.
- 4Show one Mg²⁺ and two Cl⁻ ions with square brackets and charges outside.
Answer: [Mg]²⁺ and 2 × [Cl with 8 outer electrons]⁻
Common mistakes
✗ Showing electrons shared between Mg and Cl.
✓ Ionic bonding is transfer, not sharing.
✗ Forgetting the charges on the ions in the diagram.
✓ Always show 2+ or −, and use square brackets around each ion.
✗ Drawing both Cl ions bonded together.
✓ The Cl⁻ ions are separate; each is held by attraction to Mg²⁺, not to each other.
Quick check
Try these, then reveal the answers.
What ion does aluminium form?Reveal answer
Al³⁺ (loses 3 electrons).
Why do ionic compounds conduct when molten but not solid?Reveal answer
Ions are only free to move when the lattice has melted.
What is the formula of calcium fluoride?Reveal answer
CaF₂ (Ca²⁺ + 2F⁻).
Exam tips
- Always state 'strong electrostatic forces of attraction between oppositely charged ions'.
- Use square brackets around each ion in dot-and-cross diagrams.
- Balance charges to work out formulae — total positive must equal total negative.
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Frequently asked questions
Why do atoms form ions?+
To gain a stable, full outer shell of electrons like the nearest noble gas.
Why are ionic compounds soluble in water?+
Water molecules pull the ions out of the lattice and surround them, dissolving the compound.
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