2.1.4 Trends of Period 3 Elements: 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 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 neighboring Si atoms by four 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)

Electrical conductivity

• The electrical conductivity decreases going across the Period 3 elements

Electrical Conductivity across Period 3 Elements

• Going from Na to Al, there is an increase in the number of valence electrons that are donated to the ‘sea’ of delocalised electrons
• Because of this, in Al there are more electrons available to move around through the structure when it conducts electricity, making Al a better electrical conductor than Na

• Due to the giant molecular structure of Si, there are no delocalised electrons that can freely move around within the structure
• Si is therefore not a good electrical conductor and is classified as a semimetal (metalloid)
• The lack of delocalised electrons is also why P and S cannot conduct electricity