Periodicity - Thermal Trends (Edexcel International A Level Chemistry): Revision Note

Trends in Melting & Boiling Point

Melting point

• Period 2 and 3 elements follow the same pattern in relation to their melting points

Melting Points of the Elements Across Period 3 Table

Ions of Period 3 elements with increasing positive charge (metals) and increasing of number of outer electrons across the period

• A general increase in melting point for the Period 3 elements up to silicon is observed

• Silicon has the highest melting point

• After the Si element the melting points of the elements decreases significantly

• The above trends can be explained by looking at the bonding and structure of the elements

Bonding & Structure of the Elements Table

• 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  

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

• 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 is 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 strong covalent bonds

• P, S, Cl and Ar are non-metallic elements and exist as simple molecules (P₄, S₈, Cl₂ 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

• Therefore, the melting points decrease going from P to Ar (note that the melting point of S is higher than that of P as sulphur exists as larger S₈ molecules compared to the smaller P₄ molecule)