Reactions of Period 3 elements with oxygen and water
All six elements burn when heated in oxygen. Each makes its highest normal oxide:
- 4Na + O₂ → 2Na₂O (yellow flame)
- 2Mg + O₂ → 2MgO (bright white flame)
- 4Al + 3O₂ → 2Al₂O₃ (white glow; a thin oxide layer protects aluminium)
- Si + O₂ → SiO₂ (needs strong heating)
- P₄ + 5O₂ → P₄O₁₀ (white flame, white smoke)
- S + O₂ → SO₂ (blue flame). SO₃ forms only with a catalyst: 2SO₂ + O₂ ⇌ 2SO₃.
With water: sodium reacts fast with cold water: 2Na + 2H₂O → 2NaOH + H₂ (pH about 13–14). Magnesium reacts very slowly with cold water, giving weakly alkaline Mg(OH)₂ (pH about 10). Hot magnesium reacts quickly with steam: Mg + H₂O → MgO + H₂. Aluminium, silicon, phosphorus and sulfur do not react with cold water in a simple way.
Structure and melting points of the oxides
Oxygen is very electronegative. On the left, the difference in electronegativity is big, so the oxides are ionic lattices. Moving right, the difference gets smaller and the bonds become covalent.
- Na₂O, MgO, Al₂O₃: giant ionic lattices, high melting points. MgO is higher than Na₂O because Mg²⁺ has a bigger charge and is smaller, so it attracts O²⁻ more strongly. Al₂O₃ has some covalent character because the small Al³⁺ ion distorts (polarises) the oxide ion.
- SiO₂: giant covalent (macromolecular). Every Si is bonded to 4 O; melting means breaking many strong bonds.
- P₄O₁₀, SO₂, SO₃: simple molecules. Only weak van der Waals forces (and dipole forces) act between molecules, so they melt low. P₄O₁₀ is a bigger molecule than SO₂, so it melts higher.
Acid-base nature of Period 3 oxides
Ionic oxides are basic: the oxide ion O²⁻ takes H⁺ from water to make OH⁻.
- Na₂O + H₂O → 2NaOH (pH about 14)
- MgO + H₂O → Mg(OH)₂ (only slightly soluble, pH about 9)
- They neutralise acids: MgO + 2HCl → MgCl₂ + H₂O
Covalent oxides are acidic: they react with water to make acids, or react with bases.
- P₄O₁₀ + 6H₂O → 4H₃PO₄ (pH about 0)
- SO₂ + H₂O → H₂SO₃ (pH about 2–3); SO₃ + H₂O → H₂SO₄ (pH about 0)
- SiO₂ is insoluble, but it is still acidic: it reacts with hot concentrated NaOH: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
- Neutralising a base: P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O; SO₂ + 2NaOH → Na₂SO₃ + H₂O
Amphoteric aluminium oxide
Al₂O₃ is insoluble in water but reacts with both acids and bases. That is amphoteric behaviour, which matches its mixed ionic/covalent bonding.
- As a base: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O (or with H₂SO₄ → Al₂(SO₄)₃)
- As an acid: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄ (sodium tetrahydroxoaluminate)
Try it: in the 3D, press step 4, then in free play pick each oxide and say aloud "acid, base or both?" before you read the answer.
Key formulas and definitions
- Trend across Period 3: ionic → giant covalent → simple molecular
- Na₂O + H₂O → 2NaOH | MgO + H₂O → Mg(OH)₂
- P₄O₁₀ + 6H₂O → 4H₃PO₄ | SO₃ + H₂O → H₂SO₄ | SO₂ + H₂O → H₂SO₃
- Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O | Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
- SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
Worked examples
1. Why does MgO have a higher melting point than Na₂O?
Step 1: Both are giant ionic lattices. Step 2: Mg²⁺ has a charge of 2+, Na⁺ only 1+, and Mg²⁺ is smaller. Step 3: So the attraction between Mg²⁺ and O²⁻ is stronger, and more energy is needed to separate the ions. Answer: MgO melts higher (about 2850 °C vs about 1130 °C).
2. Explain why SiO₂ has a very high melting point but SO₂ is a gas at room temperature.
Step 1: SiO₂ is giant covalent: every atom is joined by strong covalent bonds in a huge network. Step 2: To melt it, many covalent bonds must break. Step 3: SO₂ is made of small separate molecules. Melting or boiling only overcomes weak forces between molecules; no covalent bonds break. Answer: strong bonds throughout SiO₂ vs weak intermolecular forces in SO₂.
3. Write equations to show that aluminium oxide is amphoteric.
Step 1: Amphoteric means it reacts with an acid and with a base. Step 2: With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. Step 3: With base: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄. Answer: both reactions happen, so Al₂O₃ is amphoteric.
4. Predict the pH of the solution when sodium oxide and phosphorus(V) oxide are each added to water. Give equations.
Step 1: Na₂O is ionic and basic: Na₂O + H₂O → 2NaOH, a strong alkali, pH about 14. Step 2: P₄O₁₀ is molecular and acidic: P₄O₁₀ + 6H₂O → 4H₃PO₄, pH about 0–1. Answer: Na₂O pH ≈ 14; P₄O₁₀ pH ≈ 0.
5. Write an equation for the reaction of SiO₂ with sodium hydroxide and say what it shows.
Step 1: SiO₂ does not dissolve in water, so the pH test shows 7. Step 2: With hot concentrated NaOH: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O. Answer: SiO₂ reacts with a base and makes a salt and water, so it is an acidic oxide.
6. How many moles of NaOH are needed to neutralise 0.010 mol of P₄O₁₀ completely?
Step 1: P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O. Step 2: Ratio P₄O₁₀ : NaOH = 1 : 12. Step 3: 0.010 × 12 = 0.12 mol. Answer: 0.12 mol NaOH.
Common mistakes
- Saying Al₂O₃ and SiO₂ give pH 7 so they are neutral. They are insoluble; Al₂O₃ is amphoteric and SiO₂ is acidic.
- Saying SiO₂ melts high because of strong intermolecular forces. It has no separate molecules: covalent bonds must break.
- Writing SO₃ as the product of burning sulfur. Burning gives SO₂; SO₃ needs a catalyst.
- Forgetting that MgO is only slightly soluble, so its solution is weakly alkaline (about pH 9), not pH 14.