2 Properties of period 3 oxides

A survey of the properties of the oxides of Period 3 elements Understand the link between the physical properties of the highest oxides of the elements Na
S and their structure and bonding. Ionic oxides The metal oxides (Na2O, MgO, Al2O3 ) are ionic. They have high melting points. They have Ionic giant lattice structures: strong forces of attraction between ions : higher mp. They are ionic because of the large electronegativity difference between metal and O The increased charge on the cation makes the ionic forces stronger (bigger lattice enthalpies of dissociation) going from Na to Al so leading to increasing melting points. Al2O3 is ionic but does show some covalent character. This can be explained by the electronegativity difference being less big or alternatively by the small aluminium ion with a high charge being able to get close to the oxide ion and distorting the oxide charge cloud Macromolecular oxides SiO2 is Macromolecular: It has many very strong covalent bonds between atoms. High energy needed to break the many strong covalent bonds – very high mp +bp Simple molecular oxides: P4O10 (s), SO2 (g) are simple molecular with weak intermolecular forces between molecules (van der waals + permanent dipoles) so have lower mp’s. They are covalent because of the small electronegativity difference between the non-metal and O atoms. P4O10 is a molecule containing 4P’s and 10 O’s. As it is a bigger molecule and has more electrons than SO2 it will have larger van der waals forces and a higher melting point. To prove that the above compounds contain ions experimentally – melt the solids and show they conduct electricity Aluminium metal is protected from corrosion in moist air by a thin layer of aluminium oxide. The high lattice strength of aluminium oxide and its insolubility in water make this layer impermeable to air and water