Melting Point Trend (AQA A-Level Chemistry): Revision Notes

6.1.3 Melting Point Trend

Overview of Melting Point Trends in Period 3 Oxides ($Na–S$)

The melting points of the highest oxides of elements across Period 3 (sodium to sulphur) vary due to differences in bonding and structure. Generally, oxides with ionic or covalent bonds in giant lattice structures have high melting points, while those with simple molecular structures have low melting points.

Explanation of Melting Point Trends by Bonding and Structure

Metal Oxides: $\text{Na}_2\text{O}$, $\text{MgO}$, and $\text{Al}_2\text{O}_3$

Bonding: These oxides have giant ionic structures with strong electrostatic forces between the metal cations and oxygen anions.

Melting Points:

• $\text{Na}_2\text{O}$: Has a relatively high melting point due to ionic bonds between $\text{Na}^+$ and $\text{O}^{2-}$ions.
• $\text{MgO}$: Higher melting point than $\text{Na}_2\text{O}$ because $\text{Mg}^{2+}$ ions have a higher charge density, creating a stronger attraction to $\text{O}^{2-}$ ions.
• $\text{Al}_2\text{O}_3$: Despite having $\text{Al}^{3+}$ ions, it has a slightly lower melting point than expected due to some covalent character introduced by the high charge density of $\text{Al}^{3+}$, which polarises the $\text{O}^{2-}$ electron cloud.

Silicon Dioxide ($\text{SiO}_2$)

Structure: Silicon dioxide has a giant covalent structure similar to diamond, with each silicon atom covalently bonded to oxygen atoms in a 3D lattice.

Melting Point: High melting point due to the energy required to break the strong covalent bonds throughout the structure.

Non-Metal Oxides: $\text{P}_4\text{O}_{10}$ and $\text{SO}_3$

Bonding: These oxides are composed of simple molecular structures held together by weak intermolecular forces (van der Waals forces and dipole-dipole interactions).

Melting Points:

• $\text{P}_4\text{O}_{10}$: Moderate melting point as it has a larger molecular size and higher molecular mass than $\text{SO}_3$, resulting in stronger van der Waals forces.
• $\text{SO}_3$: Lower melting point due to weaker van der Waals forces in its smaller molecular structure.