Group 2 compounds
| Property | Trend down group 2 (Mg → Ba) |
|---|---|
| Solubility of hydroxides | Increases: Mg(OH)₂ is almost insoluble; Ba(OH)₂ is soluble |
| Solubility of sulphates | Decreases: MgSO₄ soluble; BaSO₄ insoluble (test for sulphate) |
| Thermal stability of carbonates and nitrates | Increases: more heat is needed to decompose BaCO₃ than MgCO₃ |
| Reactivity with water | Increases |
Why does the thermal stability increase? A small, highly charged cation such as Mg²⁺ has a high charge density and polarises (distorts) the nearby carbonate ion, weakening a C–O bond, so the carbonate decomposes easily. Larger cations polarise less.
Choose the charge and the radius of an ion. A higher charge density means more polarising power and a less stable carbonate.
| Ion | Radius (pm) | Charge density (q/r × 100) |
|---|---|---|
| Li⁺ | 76 | 1.3 |
| Na⁺ | 102 | 1.0 |
| K⁺ | 138 | 0.7 |
| Mg²⁺ | 72 | 2.8 |
| Ca²⁺ | 100 | 2.0 |
| Ba²⁺ | 135 | 1.5 |
Aluminium
Aluminium is the most abundant metal in the Earth’s crust. It is protected by a thin, tough layer of aluminium oxide, so it does not corrode. Aluminium oxide and hydroxide are amphoteric: Al(OH)₃ dissolves in acid (making Al³⁺) and in excess alkali (making the aluminate ion, [Al(OH)₄]⁻). Aluminium chloride is covalent in the vapour and dimerises to Al₂Cl₆. It is used as a catalyst (Friedel–Crafts reactions).
Group 4: tin and lead
Tin and lead each form oxidation states +2 and +4. Down the group the +2 state becomes more stable (the inert pair effect): tin(II) is a reducing agent, while lead(IV) oxide is a strong oxidising agent. Their oxides, PbO and SnO, are amphoteric. Lead compounds are poisonous, which is why lead is no longer used in petrol and paint.
| Compound | Property |
|---|---|
| SnCl₂ | Reducing agent; Sn²⁺ → Sn⁴⁺ |
| PbO₂ | Oxidising agent; used in car batteries (lead–acid) |
| PbCl₂ | White, soluble in hot water |
| PbI₂ | Bright yellow precipitate: test for lead(II) |
Key terms
- Polarisation
- Distortion of the electron cloud of an anion by a nearby cation.
- Charge density
- Charge divided by the ionic radius.
- Amphoteric
- Able to react with both acids and bases.
- Inert pair effect
- The tendency of the outer s electrons of heavy elements not to take part in bonding.
Summary
- Group 2: hydroxides become more soluble, sulphates less soluble and carbonates more stable down the group.
- Cations with high charge density polarise anions and lower thermal stability.
- Aluminium oxide and hydroxide are amphoteric.
- Tin and lead: +2 becomes more stable down the group (inert pair effect).