Skip to notes
wise otter.Chemistry
Contents
9476 / 2027
The Periodic Table

Lesson 3 of 6

Period-3 oxides and hydroxides

Track oxidation numbers, reactions with water and acid-base behaviour.

A-Level 9476 (2026-2027)

  • I can explain the specified oxide and chloride patterns.Syllabus

    Syllabus 9476, 5(e). Explain the specified oxide and chloride patterns.

Highest oxides progress from ionic bases to covalent acids

Use oxygen at -2 to check the oxidation numbers before explaining the trend.

All six specified highest oxides
OxideElement oxidation number and structureReaction with water
Na2ONa +1; ionic lattice.Na2O + H2O → 2NaOH; strongly alkaline solution.
MgOMg +2; ionic lattice.MgO + H2O → Mg(OH)2, slowly; sparing dissolution gives a weakly alkaline solution.
Al2O3Al +3; predominantly ionic extended solid with appreciable covalent character.No reaction with water under ordinary conditions.
SiO2Si +4; giant covalent network.No reaction with water under ordinary conditions.
P4O10P +5; covalent oxide, commonly represented using P4O10 molecular units.P4O10 + 6H2O → 4H3PO4; acidic solution.
SO3S +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 understanding

What is the oxidation number of phosphorus in P4O10?

  1. +10, because there are ten oxygen atoms in the formula
  2. +5, because ten O at -2 give -20, shared by four P
  3. +4, because there are four phosphorus atoms in the formula
Show the answer

+5, because ten O at -2 give -20, shared by four P

The compound is neutral, so four P atoms total +20. Each P is +5. Subscripts are not oxidation numbers.

Amphoteric aluminium compounds react on both sides

The trend is basic -> amphoteric -> acidic; water solubility is a separate question.

Acid-base reactions of the oxides
TypeExamplesRepresentative balanced reaction
BasicNa2O and MgOMgO(s) + 2H+(aq) → Mg2+(aq) + H2O(l)
Amphoteric: with acidAl2O3Al2O3(s) + 6H+(aq) → 2Al3+(aq) + 3H2O(l)
Amphoteric: with aqueous NaOHAl2O3Al2O3(s) + 2OH-(aq) + 3H2O(l) → 2[Al(OH)4]-(aq)
AcidicSiO2SiO2(s) + 2OH-(aq) → SiO32-(aq) + H2O(l), with hot concentrated alkali
AcidicP4O10P4O10 + 12OH- → 4PO43- + 6H2O, for complete neutralisation
AcidicSO3SO3 + 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.

A white aluminium hydroxide precipitate can dissolve two ways
  1. Add acid

    Al(OH)3(s) + 3H+(aq) → Al3+(aq) + 3H2O(l).

  2. Add excess aqueous NaOH

    Al(OH)3(s) + OH-(aq) → [Al(OH)4]-(aq).

  3. 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 exampleCheck atoms and charge in an amphoteric equation

Why does Al2O3 + 2OH- → 2[Al(OH)4]- need water?

Try it first. Then show the worked stepsHide the worked steps
  1. The products contain eight O atoms and eight H atoms.
  2. Al2O3 plus 2OH- supplies five O and two H.
  3. Add three H2O to the left: this supplies the missing three O and six H. Total charge remains -2 on each side.
Answer

Al2O3 + 2OH- + 3H2O → 2[Al(OH)4]-.

Check your understanding

Which equation shows aluminium oxide acting as an acidic oxide?

  1. Al2O3 + 6H+ -> 2Al3+ + 3H2O
  2. Al2O3 + 2OH- + 3H2O -> 2[Al(OH)4]-
  3. Al2O3 + 3H2O -> 2Al(OH)3
Show the answer

Al2O3 + 2OH- + 3H2O -> 2[Al(OH)4]-

Reacting with a base shows acidic behaviour. Al2O3 also reacts with acid (Al2O3 + 6H+ -> 2Al3+ + 3H2O), so it is amphoteric.