13.1.1 Explain the physical states (under standard conditions) and electrical conductivity (in the molten state) of chlorides and oxides of the elements in Period 3 in terms of their bonding and structure.

Physical Trends:

• Overall decrease in melting and boiling points as the structures change from ionic to simple molecular.  This happens between Mg and Al; metallic chlorides (NaCl, MgCl₂, AlCl₃ ) are solids, non-metal/molecular 
• chlorides are gases (covalent bonding between atoms within molecules but Van Der Waal’s between molecules).  No giant covalent chloride as silicon tetrachloride is a gas;

Conductance:

• good in molten ionic chlorides while poor in the simple molecular chlorides.

Chemical trends

·     the type of bonding formed changes from ionic to covalent;  aluminium trichloride however is now covalently bonded instead of ionic;

·     no chloride for argon as it does not form any compounds as it is a noble gas with full outer shell.

·     there is a regular trend in the empirical formula of the highest chlorides because of the increase in oxidation state from +1 to +5;

     

            NaCl      MgCl₂     Al₂ Cl₆ (=dimer)       SiCl₄         PCl₅        

·     some elements show variable oxidation states; e.g. PCl₅  and PCl₃.

Physical state:

• melting and boiling points: rise to a maximum at SiO₂ and then decline
• ionic compounds (Na₂O, MgO and Al₂O₃) on the left side have relatively high melting and boiling points (high lattice energy); solids at room temperature;
• giant covalent molecular structure like silicon dioxide has the highest melting and boiling points (lattice held together by strongest bond i.e. covalent); solids at room temperature;
• lowest melting and boiling points are for the simple molecular structures (starting from P₄O₆); they are gases at room temperature (weaker dipole-dipole attractions).

Electrical conductivity in molten state:

• changes from good to semi-conductor to poor; this is the case because their structure changes from ionic to simple molecular. 
• None of the conductors will conduct as solids (because no free moving electrons).

Chemical trends:

• the type of bonding between element and oxygen changes from ionic to covalent;  aluminium oxide is still ionic but has quite a strong covalent character whilst silicon dioxide is mostly covalent;
• type of oxide:

_o    basic:   Na₂O      MgO    
_o    amphoteric:  Al₂ O_3 
o    acidic:  SiO₂         P₄ O₁₀        SO₃     Cl₂ O₇ 

• some elements show variable oxidation states; e.g.  P₄ O₁₀ and P₄ O₆ and  SO₃ and  SO₂;
• trend in empirical formula of highest oxide: ratio of  element : oxygen  (X : O) increases from 0.5 to 3.5.

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