Melting Point Trend (AQA A Level Chemistry)

• 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 lattice structures and thus, high melting points

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