Melting Point of Period 3 Oxides

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

230626 melting point 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.

230626 ionic bond with covalent character in Al2O3

Al3+ has high charge density and strong polarising power.

It is able to distort electron cloud of O2- 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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