Period 3 Chlorides & Oxides (CIE A Level Chemistry)

Period 3 chlorides

• The bonding and structure of the Period 3 elements are summarised in the table below:

• 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
• 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 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)
• The presence of a ‘sea’ of delocalised electrons also determines whether the element is a good conductor or not
• Going across the period the electrical conductivity of the elements decreases due to lack of delocalised electrons

• The electronegativities of the Period 3 elements therefore determines the chemical bonding and structure of their chlorides and oxides

• Going across Period 3, their chlorides and oxides become more covalent and their structure shifts from a giant ionic to a simple molecular structure
• Their reactions with water become more vigorous as a result of this as it becomes easier to hydrolyse the chlorides and oxides