3.2.4 Period 3 Elements and Oxides

Sodium reacts vigorously with cold water to form NaOH and H₂. Magnesium reacts only slowly with cold water but reacts readily with steam to form MgO and H₂.

Write the products for the condition actually given. The contrast is useful: sodium readily forms the hydroxide with cold water, whereas hot steam drives magnesium to the oxide.

Know the oxide set explicitly: Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃. Their structures change from ionic solids through giant covalent SiO₂ to molecular phosphorus and sulfur oxides.

Use that list as the spine for the topic because the later melting-point and acid–base questions depend on knowing which oxide you actually have.

The highest oxides have high melting points from Na₂O through Al₂O₃, with SiO₂ also very high because it is giant covalent. The values then fall sharply for molecular P₄O₁₀, SO₂ and SO₃.

Describe the broad pattern first. The detailed explanation belongs to the structure: ionic lattices and giant covalent networks require much more energy to disrupt than intermolecular attractions between molecules.

For these Period 3 oxides, I’d learn the water reactions as a compact map: Na₂O gives NaOH; MgO reacts only slowly or slightly to give Mg(OH)₂; Al₂O₃ and SiO₂ do not react with water under ordinary conditions; P₄O₁₀, SO₂ and SO₃ give acidic solutions.

This is one of those lines where a compact reaction map is worth learning. Keep “does not react with water” separate from “is not acidic/basic in other reactions”.

P₄O₁₀ + water gives phosphoric(V) acid, H₃PO₄; SO₂ + water gives sulfurous acid chemistry associated with sulfite(IV) species; SO₃ + water gives H₂SO₄ and sulfate(VI) chemistry.

Use the sulfur oxidation state to keep the two families straight: S(IV) goes with sulfite chemistry and S(VI) with sulfate chemistry. If the question switches from the acid to the anion, carry that oxidation-state label across.

When you explain Period 3 oxide melting points, start by identifying the structure. Na₂O, MgO and Al₂O₃ are ionic, SiO₂ is giant covalent, and those structures require strong attractions or bonds to be overcome, so their melting points are high.

P₄O₁₀, SO₂ and SO₃ are molecular, so melting only requires intermolecular forces to be overcome. Tie every explanation to the actual structure; “strong bonds” is incomplete if those bonds are inside molecules and not broken on melting.

Look across Period 3: the oxide chemistry moves broadly from basic ionic oxides through amphoteric Al₂O₃ to acidic covalent oxides. Use their reactions with water, acids and bases as the evidence for that change.

Al₂O₃ is the useful turning point because it is amphoteric. SiO₂ is acidic even though it does not react with water: its reaction with strong base is the evidence, so do not equate “acidic oxide” with “forms an acid simply by adding water”.

Use equations to demonstrate acid–base character. Basic Na₂O or MgO reacts with acids; amphoteric Al₂O₃ reacts with both acids and bases; acidic SiO₂ and the phosphorus/sulfur oxides react with suitable bases.

Balance the equation for the oxide actually named. If aluminium oxide is involved, showing both an acid reaction and a base reaction is what establishes amphoteric behaviour.