Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.1Advanced Inorganic Chemistry

Properties of Period 3 Elements and their Oxides — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 1 of 3

📚 AQA A Level Chemistry Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

Period 3 Elements: Reactions, Oxides and Melting Points — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Describe and explain trends in reactions of Period 3 elements with water and oxygen, in oxide structure and acid-base behaviour, and in melting points.Sodium reacts vigorously …

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Amphoteric
  • Giant covalent

Part 1 of 5Sodium and magnesium with water

Sodium reacts vigorously with cold water, floating, fizzing and melting into a ball: 2Na + 2H2O → 2NaOH + H2.

Sodium and magnesium with water
The solution is strongly alkaline (about pH 14).
Magnesium reacts very slowly with cold water to give Mg(OH)2 (sparingly soluble, pH about 9),
  • but burns in steam to give magnesium oxide and hydrogen: Mg + H2O(g) → MgO + H2.
Sodium is more reactive because it has a lower first ionisation energy (one electron lost, larger atom, less nuclear attraction) than magnesium,
  • which must lose two electrons.

Part 2 of 5Reaction with oxygen and oxide properties

Na burns with a yellow flame to give Na2O; Mg burns with a bright white flame to give MgO; Al forms a protective Al2O3 layer; Si gives SiO2; P burns to P4O10 (and P4O6 in limited oxygen); S burns with a blue flame to SO2.

OxideStructure and bondingAcid-base characterWith water
Na2Ogiant ionicbasicNaOH, pH about 14
MgOgiant ionicbasicMg(OH)2, pH about 9
Al2O3ionic with covalent characteramphotericinsoluble
SiO2giant covalentweakly acidicinsoluble
P4O10simple molecularacidicH3PO4, pH 0 to 1
SO2, SO3simple molecularacidicH2SO3, H2SO4

Basic oxides contain O2-, which reacts with water to give OH-. Amphoteric Al2O3 reacts with both acids (forming Al3+ salts) and bases (forming aluminate, e.g. [Al(OH)4]-).

Part 3 of 5Melting point trend

Melting points rise from Na to Al as metallic bonding strengthens (more delocalised electrons, higher ionic charge, smaller ions), peak at silicon (a giant covalent lattice with many strong covalent bonds to break), then fall sharply for the simple molecular elements. Among these, sulfur (S8) is higher than phosphorus (P4) because larger molecules have stronger London forces, while chlorine (Cl2) and argon (single atoms) are lowest.

For the oxides, the giant ionic oxides (Na2O, MgO, Al2O3) and giant covalent SiO2 have high melting points, while the molecular oxides of P and S have low ones.

Part 4 of 5Linking structure to behaviour

The change from basic to acidic oxides follows the change from ionic to covalent bonding across the period.

Linking structure to behaviour
Metal oxides (ionic, containing O2-) neutralise acids; non-metal oxides (covalent) react with water or alkali to give acidic species.
Aluminium sits at the boundary, which explains its amphoteric oxide.
When predicting an oxide’s pH in water, first decide its structure, then its acid-base type, then write the equation.

Part 5 of 5Worked examples

  1. Write an equation for the reaction of sodium oxide with water and give the pH. [2]2 marks

  2. Explain why the melting point of silicon dioxide is far higher than that of sulfur dioxide. [3]3 marks

  3. Write equations showing Al2O3 is amphoteric. [2]2 marks

  4. Write the equation for phosphorus(V) oxide with water and state the pH. [2]2 marks

Key terms

TermMeaning
AmphotericAble to react with both acids and bases.
Giant covalentLattice of atoms joined by covalent bonds.
Basic oxideOxide that neutralises acids, containing O2−.
Acidic oxideOxide reacting with water or bases to give acidic species.
Ionisation energyEnergy to remove one electron from each atom of a gaseous element.

Avoid these mistakes

1
Saying magnesium does not react with water - it reacts slowly with cold water and readily with steam.
2
Writing P2O5 as the molecule - the real molecule is P4O10.
3
Explaining the sulfur versus phosphorus melting points by bond strength instead of London forces.
4
Saying SiO2 melts by overcoming intermolecular forces - it has covalent bonds throughout.

Last updated: