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 oxidised 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