Melting Point Trend (AQA A Level Chemistry): Revision Note

Melting Point Trend

• The melting points of the oxides vary significantly across Period 3 and show a clear trend

• Melting point is a clear indication of the forces of attraction which exist between ions, atoms or molecules

A graph to show the melting points of the Period 3 Oxides

Ionic Oxides

• The graph starts off showing the melting points of the ionic oxides

  • Sodium oxide

  • Magnesium oxide

  • Aluminium oxide

• These are ionic oxides because the bonding exists between metals and non-metals

• They have giant ionic lattice structures

  • So, large amounts of energy are needed to overcome the strong electrostatic attractions between oppositely charged ions

  • This means that they have high melting points

• Going from Na₂O to MgO, the melting point increases significantly:

  • Mg²⁺ has a greater charge (+2) and smaller ionic radius than Na⁺

  • This gives Mg²⁺ a higher charge density

  • The electrostatic attraction between Mg²⁺ and O^2- is therefore stronger than between Na⁺ and O^2-, resulting in a higher lattice energy and higher melting point

Giant Covalent Oxides

• The graph then shows a giant covalent oxide

  • Silicon dioxide

• This is covalent because both silicon and oxygen are non metals

• The millions of covalent bonds within this giant structure are extremely strong, and thus it has a high melting point

• Giant covalent structures can also be called macromolecules or giant molecules

Simple Covalent Oxides

• The graph then shows a significant drop in melting point, as we reach the simple covalent oxide molecules

  • Phosphorus(V) oxide

  • Sulfur dioxide

  • Sulfur trioxide

• These are small molecules with only weak intermolecular forces of attraction between them

• Sulfur dioxide and sulfur trioxide are both gases at room temperature, because both their melting point and boiling point are so low

  • Sulfur trioxide, SO₃, has a slightly higher melting point than sulfur dioxide, SO₂, because of the increase in intermolecular forces between the slightly larger SO₃ molecules

Summary Table of the Physical Properties of the Period 3 Oxides