Period 3 Elements and Their Oxides

This lesson focuses on the reactions of Period 3 elements with oxygen and the acid–base character of the resulting oxides. This is a key part of AQA specification 3.2.4 and links structure and bonding to chemical reactivity. Understanding the trend from basic to amphoteric to acidic oxides is essential.

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Reactions of Period 3 Elements with Oxygen

Sodium (Na)

4Na(s) + O₂(g) → 2Na₂O(s)

• Burns with a bright yellow-orange flame.
• Produces a white solid, sodium oxide.
• Sodium can also form the peroxide Na₂O₂ if excess oxygen is present.

Magnesium (Mg)

2Mg(s) + O₂(g) → 2MgO(s)

• Burns with a brilliant white flame.
• Produces a white powder, magnesium oxide.
• Very exothermic reaction.

Aluminium (Al)

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

• Burns with a brilliant white flame when powdered.
• Produces a white solid, aluminium oxide.
• Bulk aluminium is protected by a tough, thin oxide layer that prevents further reaction (this is why aluminium appears unreactive despite being high in the reactivity series).

Silicon (Si)

Si(s) + O₂(g) → SiO₂(s)

• Reacts only at very high temperatures.
• Produces silicon dioxide (silica), a giant covalent solid.
• SiO₂ has a very high melting point (1713 °C) and is the main component of sand.

Phosphorus (P)

P₄(s) + 5O₂(g) → P₄O₁₀(s)

• White phosphorus burns spontaneously in air with a bright white flame.
• Produces phosphorus(V) oxide, P₄O₁₀, a white solid.
• In limited oxygen: P₄(s) + 3O₂(g) → P₄O₆(s) (phosphorus(III) oxide).

Sulfur (S)

S(s) + O₂(g) → SO₂(g)

• Burns with a blue flame.
• Produces sulfur dioxide (SO₂), a colourless, choking gas.
• SO₂ can be further oxidised to SO₃ in the presence of a catalyst (V₂O₅, as in the Contact process): 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

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Acid–Base Character of Period 3 Oxides

Oxide	Formula	Bonding	Acid–base character
Sodium oxide	Na₂O	Ionic	Strongly basic
Magnesium oxide	MgO	Ionic	Basic
Aluminium oxide	Al₂O₃	Ionic with covalent character	Amphoteric
Silicon dioxide	SiO₂	Giant covalent	Weakly acidic
Phosphorus(V) oxide	P₄O₁₀	Simple molecular (covalent)	Acidic
Sulfur trioxide	SO₃	Simple molecular (covalent)	Strongly acidic
Dichlorine heptoxide	Cl₂O₇	Simple molecular (covalent)	Strongly acidic

The Trend

Moving left to right across Period 3, the oxides change from basic (metal oxides) through amphoteric (Al₂O₃) to acidic (non-metal oxides). This is because:

• Metal oxides contain oxide ions (O²⁻) that act as bases (proton acceptors or react with water to form OH⁻).
• Non-metal oxides have covalent bonds to oxygen; when dissolved in water, they react to form acidic solutions (oxyacids).

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Reactions of Oxides with Water

Na₂O (strongly basic)