Ionic bonding and ionic compounds
Ionic bonds form between metals and non-metals. The metal transfers electrons to the non-metal; the resulting oppositely charged ions attract each other strongly.
Forming ions — predicting from groups
| Group | Ion charge |
|---|---|
| 1 | +1 (e.g. Na⁺) |
| 2 | +2 (e.g. Mg²⁺) |
| 3 | +3 (e.g. Al³⁺) |
| 5 | −3 (e.g. N³⁻) |
| 6 | −2 (e.g. O²⁻) |
| 7 | −1 (e.g. Cl⁻) |
| 0 | None — no ions formed |
Group number for metals = positive charge; (8 − group number) for non-metals = negative charge.
Working out a formula — balance the charges
To find the formula of an ionic compound, write down the ion of each element, then add ions until total positive = total negative.
Examples:
- Na⁺ + Cl⁻ → NaCl
- Mg²⁺ + 2 Cl⁻ → MgCl₂
- Al³⁺ + 3 OH⁻ → Al(OH)₃
- 2 Na⁺ + S²⁻ → Na₂S
- 3 Mg²⁺ + 2 N³⁻ → Mg₃N₂
The lowercase number tells you how many of each ion. Compounds are electrically neutral — total + = total −.
Dot-and-cross diagrams (electron-transfer style)
For an ionic compound:
- Show the outer shell of each atom before bonding (use dots for one atom, crosses for the other).
- Show the electron(s) transferring from the metal to the non-metal.
- Show the resulting ions with charges in square brackets, e.g. [Na]⁺ and [:Cl:]⁻.
For NaCl: one electron moves from Na's outer shell to Cl's outer shell; you draw [Na]⁺ (no outer electrons in shell 3) and [Cl]⁻ with 8 electrons in shell 3.
Giant ionic lattice
Solid ionic compounds aren't molecules — they form a 3D regular arrangement called a giant ionic lattice. Each Na⁺ is surrounded by 6 Cl⁻ ions and vice versa.
The whole lattice is held together by very strong electrostatic forces between oppositely charged ions, in all directions.
Properties of ionic compounds (the standard exam answer)
- High melting and boiling points — many strong ionic bonds throughout the lattice; lots of energy needed to break them.
- Brittle — when the lattice is forced, ions of like charge can be pushed next to each other and repel, splitting the crystal.
- Conduct electricity when molten or dissolved — ions are free to move and carry charge.
- Do NOT conduct when solid — ions are locked in place.
- Often soluble in water — water molecules separate the ions.
✦Worked example— Worked example — magnesium oxide
Mg (2,8,2) loses 2 electrons → Mg²⁺ (2,8). O (2,6) gains 2 electrons → O²⁻ (2,8). Compound: MgO — a giant ionic lattice with very high melting point (~2850 °C, hence its use as a refractory material in furnaces).
⚠Common mistakes
- Saying NaCl exists as molecules. It exists as a lattice, not molecules.
- Drawing wrong charge. Group 1 → +1; Group 7 → −1; etc.
- Forgetting brackets and charges on dot-and-cross diagrams of ions.
- Saying ionic compounds always conduct. They only conduct when molten or dissolved.
Links
Builds on C1.4 (electronic structure). Extends to C2.6 (properties from structure), C4 (acids react with bases to give ionic salts) and C8 (tests using ions).
AI-generated · claude-opus-4-7 · v3-deep-chemistry