Ionic Bonding
What is Ionic Bonding?
Ionic bonding occurs between a metal and a non-metal. Electrons are transferred from the metal atom to the non-metal atom. This creates oppositely charged ions that attract each other — the electrostatic attraction between these ions is the ionic bond.
Key principle: Atoms are most stable when they have a full outer electron shell (like a noble gas). Metal atoms lose electrons to achieve this; non-metal atoms gain electrons.
Formation of Ions
Metals (left of periodic table) have 1–3 electrons in their outer shell → lose electrons → form positive ions (cations).
- Sodium: 2,8,1 → loses 1 electron → Na⁺ (2,8) — same as neon
- Calcium: 2,8,8,2 → loses 2 electrons → Ca²⁺ (2,8,8)
- Aluminium: 2,8,3 → loses 3 electrons → Al³⁺ (2,8)
Non-metals (right of periodic table) have 5–7 electrons in outer shell → gain electrons → form negative ions (anions).
- Chlorine: 2,8,7 → gains 1 electron → Cl⁻ (2,8,8) — same as argon
- Oxygen: 2,6 → gains 2 electrons → O²⁻ (2,8)
Dot-and-Cross Diagrams
Dot-and-cross diagrams show the transfer of electrons:
- Outer shell electrons only are shown
- Use dots for electrons from one atom, crosses for electrons from the other
- Show the resulting ions with charges in square brackets
Example — NaCl: Na (2,8,1) transfers 1 electron to Cl (2,8,7) → Na⁺ [2,8] and Cl⁻ [2,8,8]
Example — MgO: Mg (2,8,2) transfers 2 electrons to O (2,6) → Mg²⁺ [2,8] and O²⁻ [2,8]
The Ionic Lattice
Ionic compounds do not exist as simple molecules. Instead, they form giant ionic lattice structures — a regular, 3D arrangement of alternating positive and negative ions, held together by strong electrostatic forces (attractions between opposite charges in ALL directions).
Example: In NaCl, each Na⁺ is surrounded by 6 Cl⁻ ions, and each Cl⁻ is surrounded by 6 Na⁺ ions.
Properties of Ionic Compounds
| Property | Explanation |
|---|---|
| High melting and boiling point | Strong electrostatic forces throughout the giant lattice → large amounts of energy needed to break them → melt at very high temperatures |
| Hard and brittle | Hard because ions are held rigidly; brittle because when a force is applied, like-charged ions are forced next to each other → repulsion → lattice shatters |
| Conduct electricity when molten or dissolved in water | Ions are free to move and carry charge; in solid state, ions cannot move → do not conduct |
| Soluble in water (many) | Water molecules can surround individual ions and pull them away from the lattice (hydration) |
| Do NOT conduct electricity when solid | Ions are held in fixed positions in the lattice — cannot move to carry charge |
Common Ionic Compound Formulae
Working out formulae: balance the charges so the compound is overall neutral.
- NaCl: Na⁺ + Cl⁻ → 1:1 ratio → NaCl
- MgCl₂: Mg²⁺ + 2Cl⁻ → 1:2 ratio → MgCl₂
- Al₂O₃: 2Al³⁺ + 3O²⁻ → 2:3 ratio → Al₂O₃
WJEC Exam Tip
WJEC Eduqas past papers regularly ask for:
- Dot-and-cross diagrams for ionic bonding (draw with squares/brackets + charge)
- Explanation of ionic compound properties in terms of structure
- Why ionic compounds conduct electricity when molten/dissolved but not solid
AI-generated · claude-opus-4-7 · v3-wjec-combined-science