Melting Point of Period 3 Oxides

Let's consider the melting points of Period 3 oxides.

1. Metal Oxides (Na₂O to Al₂O₃)

The metal oxides are ionic compounds with strong electrostatic attraction between oppositely charged ions held in giant ionic lattice.

Hence their melting points are high.

Comparing their melting points:

MgO has higher melting point than Na₂O due to higher charge Mg²⁺, magnitude of lattice energy for MgO is greater, hence stronger ionic bonds.

Al₂O₃ has lower melting point than MgO even though Al³⁺ has a higher charge than Mg²⁺.

This anomaly in melting point of ionic compounds can be explained via the concept of ionic bond with covalent character.

Al³⁺ has high charge density and strong polarising power.

It is able to distort electron cloud of O^2- to a significant extent, hence Al₂O₃ has ionic bond with significant covalent character.

This gives Al₂O₃ properties of simple covalent compounds hence its melting point is lower than MgO.

Note that Al₂O₃ still has a higher melting point than Na₂O.

2. SiO₂

SiO₂ is a giant covalent compound with strong and extensive covalent bonds hence it has high melting point.

We need to remember melting point of SiO₂ is between Na₂O and Al₂O₃.

However we do not need to explain this since comparing strength of ionic bond and covalent bond is not in syllabus.

3. Non-metal Oxides (P₄O₁₀ and SO₃)

P₄O₁₀ and SO₃ are simple molecules with weak forces of attraction between molecules, hence melting points are much lower.

Comparing their melting points:

P₄O₁₀ has a bigger and more polarisable electron cloud, stronger id-id attraction between molecules hence higher melting point than SO₃.

Note P₄O₁₀ is a solid and SO₃ is a gas under standard conditions which also shows that attraction between P₄O₁₀ molecules are stronger.

Topic: Periodicity, Inorganic Chemistry, A Level Chemistry, Singapore

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