Trends of Period 3 Elements: Melting Point (AQA A-Level Chemistry): Revision Notes

2.1.4 Trends of Period 3 Elements: Melting Point

Explanation of Melting Point Trend

The melting points of the elements in Period 3 vary significantly, and the changes are closely related to the structure and bonding present in each element. The melting points first increase, then reach a peak, and finally decrease sharply.

Metallic Elements (Sodium to Aluminium)

The melting points of the metallic elements (sodium $Na$, magnesium $Mg$, and aluminium $Al$) increase across the period.

Reasons

• Number of Outer Electrons: From $Na$ to $Al$, the number of delocalised electrons available to form the metallic bond increases. This leads to stronger metallic bonding.
• Size of Positive Ions: As you move across the period, the size of the positive metal ions decreases. Smaller ions create stronger attractions between the positive ions and the delocalised electrons.
• As a result, more energy is required to overcome these stronger metallic bonds, leading to higher melting points.

Silicon ($Si$) – A Giant Covalent Structure

The melting point of silicon is the highest in Period 3 because it has a giant covalent structure.

Reasons

• Silicon forms a giant covalent lattice, where all atoms are bonded together by strong covalent bonds.
• A large amount of energy is required to break these covalent bonds, leading to a very high melting point for silicon.

Phosphorus, Sulphur, Chlorine, and Argon (Non-Metals)

The melting points of the non-metals (phosphorus $P_4$, sulfur $S_8$, chlorine $Cl_2$, and argon $Ar$) are much lower compared to silicon.

Reasons

• These elements exist as simple covalent molecules (except for argon, which exists as single atoms). The forces between the molecules or atoms are weak van der Waals' forces, which require little energy to overcome.
• Phosphorus ($P_4$), sulfur ($S_8$), and chlorine ($Cl_2$) are all molecular substances, and their melting points depend on the strength of the van der Waals' forces between their molecules.
  • Sulfur ($S_8$) has the highest melting point of the non-metals because it has the largest molecules and the greatest number of electrons, leading to stronger van der Waals' forces.
  • Phosphorus ($P_4$) has fewer electrons and a lower melting point than sulphur.
  • Chlorine ($Cl_2$) has even fewer electrons and weaker van der Waals' forces, resulting in a lower melting point.
• Argon ($Ar$) exists as individual atoms with very weak van der Waals' forces, giving it the lowest melting point in the period.