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

Trend: Melting Point

Melting points of the elements across period 3

• A pattern is a little harder to see from the data, but you can see that it rises and falls:

Melting points of the period 3 elements

• The trends in melting point can be explained by looking at the bonding and structure of the elements

Bonding & Structure of the Period 3 Elements

• The table shows that Na, Mg, and Al are metallic elements which form positive ions arranged in a giant lattice in which the ions are held together by a 'sea' of delocalised electrons around them.

Metal cations form a giant lattice held together by electrons that can move around freely

• The electrons in the ‘sea’ of delocalised electrons are those from the valence shell of the atoms

• Na will donate one electron into the ‘sea’ of delocalised electrons, Mg will donate two, and Al three electrons

• As a result of this, the metallic bonding in Al is stronger than in Na

• This is because the electrostatic forces between a 3+ ion and the larger number of negatively charged delocalised electrons are much larger compared to a 1+ ion and the smaller number of delocalised electrons in Na

• Because of this, the melting points increase going from Na to Al

• Si has the highest melting point due to its giant molecular structure, in which each Si atom is held to its neighbouring Si atoms by four strong covalent bonds

• P, S, Cl, and Ar are non-metallic elements and exist as simple molecules (P₄, S₈, Cl_2, and Ar as a single atom)

• The covalent bonds within the molecules are strong; however, between the molecules, there are only weak instantaneous dipole-induced dipole forces

• It doesn’t take much energy to break these intermolecular forces

• Among the molecular elements, S₈ has the highest melting point (the largest molecule), then P₄, with Cl₂ and Ar much lower

Related topics

• Atomic Radius

• First Ionisation Energy

• Structure & Bonding