# 2.3 Ionic bonding and Metallic bonding

F.4 Chemistry · Topic II Microscopic World (I) · Lesson 1

Why some substances conduct electricity, and the two kinds of bond that hold metals and ionic compounds together.

## Key points

- **Conductors** (all metals, graphite) conduct in both solid and liquid states. **Electrolytes** conduct only when molten (ionic compounds) or in aqueous solution (ionic compounds, acids, alkalis), and are decomposed. **Non-conductors** never conduct.
- **Metallic bond ／ 金屬鍵**: the strong, non-directional attraction between metal cations and the 'sea' of **delocalized electrons ／ 離域電子**.
- **Ionic bond ／ 離子鍵**: the strong electrostatic attraction between cations and anions, formed after electrons are **transferred** from metal atoms to non-metal atoms.
- Ions copy a noble-gas electronic arrangement: metal atoms lose 1–3 electrons → **cations**; non-metal atoms gain 1–3 electrons → **anions**.
- **Criss-cross method**: swap the charge numbers into subscripts, then simplify. Al^3+^ and O^2−^ → Al~2~O~3~; Mg^2+^ and O^2−^ → MgO. Cation always first.
- Evidence for ions: on wet filter paper, coloured ions **migrate** towards the oppositely charged electrode.

## Must know

| Ion | Colour | Ion | Colour |
|---|---|---|---|
| Cu^2+^ | Blue | CrO~4~^2−^ | Yellow |
| Fe^2+^ | Pale green | Cr~2~O~7~^2−^ | Orange |
| Fe^3+^ | Yellow-brown | MnO~4~^−^ | Purple |
| Co^2+^ | Pink | Mn^2+^ | Very pale pink |
| Ni^2+^ | Green | Cr^3+^ | Green |

Polyatomic ions to memorise: NH~4~^+^ ammonium · OH^−^ hydroxide · NO~3~^−^ nitrate · SO~4~^2−^ sulphate · CO~3~^2−^ carbonate · HCO~3~^−^ hydrogencarbonate · PO~4~^3−^ phosphate.

## Common mistakes

- ✗ "A metallic bond holds metal **atoms** together." ✓ It is between metal **ions** and the sea of delocalized electrons.
- ✗ "A molecule of NaCl." ✓ Ionic compounds have no molecules. NaCl is a giant ionic structure; the formula only gives the ratio of ions.
- ✗ "Solid NaCl does not conduct because it has no ions." ✓ It is full of ions, but they are fixed in the lattice. They move only when molten or dissolved.
- ✗ "Adding acid to chromate turns the solution green." ✓ Yellow CrO~4~^2−^ → orange Cr~2~O~7~^2−^.
- ✗ "NH~4~Cl is not ionic because it has no metal." ✓ NH~4~^+^ is a cation, so ammonium salts are ionic compounds.

## Quick check

1. Why does molten sodium chloride conduct electricity while solid sodium chloride does not?
2. Write the chemical formula of aluminium sulphate.
3. What colour change is seen when dilute sulphuric acid is added to potassium chromate solution?

## Answers

1. Only when molten are the Na^+^ and Cl^−^ ions free to move and carry the current; in the solid they are held in fixed positions.
2. Al~2~(SO~4~)~3~
3. Yellow → orange (chromate ions become dichromate ions).

## Did you know?

Ruby and sapphire are the same mineral, corundum (Al~2~O~3~). A trace of chromium(III) ions makes it red ruby; traces of iron and titanium ions make it blue sapphire. Pure corundum is colourless.