AQA A Level Chemistry

Revision Notes

6.1.3 Melting Point Trend

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 lattice structures and thus, high melting points

Giant Covalent Oxides

  • The graph then shows a giant covalent oxide
    • Sulfur dioxide
  • This is covalent because both sulfur 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

Exam Tip

Make sure that you can state and explain the link between the physical properties of the Period 3 Oxides and their structure and bonding!


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