Topic 3 of 6
Period-3 oxides and hydroxides
Track oxidation numbers, reactions with water and acid-base behaviour.
A-Level 9476 (2026-2027)
Highest oxides progress from ionic bases to covalent acids
Use oxygen at -2 to check the oxidation numbers before explaining the trend.
| Oxide | Element oxidation number and structure | Reaction with water |
|---|---|---|
| Na2O | Na +1; ionic lattice. | Na2O + H2O → 2NaOH; strongly alkaline solution. |
| MgO | Mg +2; ionic lattice. | MgO + H2O → Mg(OH)2, slowly; sparing dissolution gives a weakly alkaline solution. |
| Al2O3 | Al +3; predominantly ionic extended solid with appreciable covalent character. | No reaction with water under ordinary conditions. |
| SiO2 | Si +4; giant covalent network. | No reaction with water under ordinary conditions. |
| P4O10 | P +5; covalent oxide, commonly represented using P4O10 molecular units. | P4O10 + 6H2O → 4H3PO4; acidic solution. |
| SO3 | S +6; covalent oxide; molecular description is useful for its vapour. | SO3 + H2O → H2SO4; vigorous hydration gives acid. |
The highest positive oxidation numbers in these oxides rise with the number of valence electrons available for bonding. These are formal oxidation numbers, not necessarily actual ionic charges: SiO2 is not a lattice of bare Si4+ and O2- ions.
Across the period, increasing element electronegativity reduces the electronegativity difference from oxygen. Bonding changes from predominantly ionic towards covalent. Oxygen in an ionic basic oxide can accept protons; covalent non-metal oxides instead commonly form oxoacids or react with bases. Silicon dioxide remains insoluble because its extended network is difficult to disrupt, despite being classified as acidic.
Check your understandingWhy is phosphorus +5 in P4O10 rather than +10?Think it through, then reveal the answer
Amphoteric aluminium compounds react on both sides
The trend is basic -> amphoteric -> acidic; water solubility is a separate question.
| Type | Examples | Representative balanced reaction |
|---|---|---|
| Basic | Na2O and MgO | MgO(s) + 2H+(aq) → Mg2+(aq) + H2O(l) |
| Amphoteric: with acid | Al2O3 | Al2O3(s) + 6H+(aq) → 2Al3+(aq) + 3H2O(l) |
| Amphoteric: with aqueous NaOH | Al2O3 | Al2O3(s) + 2OH-(aq) + 3H2O(l) → 2[Al(OH)4]-(aq) |
| Acidic | SiO2 | SiO2(s) + 2OH-(aq) → SiO32-(aq) + H2O(l), with hot concentrated alkali |
| Acidic | P4O10 | P4O10 + 12OH- → 4PO43- + 6H2O, for complete neutralisation |
| Acidic | SO3 | SO3 + 2OH- → SO42- + H2O |
NaOH is a soluble strong base. Mg(OH)2 is basic but sparingly soluble: limited solubility should not be confused with partial dissociation of the small amount that dissolves. Both react with acids. Al(OH)3 is amphoteric and can dissolve in either acid or excess aqueous sodium hydroxide.
- Add acid
Al(OH)3(s) + 3H+(aq) → Al3+(aq) + 3H2O(l).
- Add excess aqueous NaOH
Al(OH)3(s) + OH-(aq) → [Al(OH)4]-(aq).
- Name the evidence
Dissolution in both acid and alkali supports amphoteric behaviour; the sodium ions are spectators in the second equation.
The familiar acidic classification of SiO2 refers to its reaction with bases, not to dissolution in water. Likewise, no observable reaction with dilute hydrochloric acid is not evidence that a solid is neutral. Distinguish solubility, speed of reaction and acid-base character.
Worked example
Check atoms and charge in an amphoteric equation
Why does Al2O3 + 2OH- → 2[Al(OH)4]- need water?
- The products contain eight O atoms and eight H atoms.
- Al2O3 plus 2OH- supplies five O and two H.
- Add three H2O to the left: this supplies the missing three O and six H. Total charge remains -2 on each side.
Al2O3 + 2OH- + 3H2O → 2[Al(OH)4]-.