13.1- Periodic trends in the third period
13.1.1 - Explain the physical properties of the chlorides and oxides of the elements in the third period (Na to Ar) in terms of their bonding and structure. Include the following oxides and chlorides: Oxides: Na2O, MgO, Al2O3, SiO2, P4O6 and P4O10, SO2 and SO3, Cl2O and Cl2O7. Chlorides: NaCl, MgCl2, Al2Cl6, SiCl4, PCl3 and PCl5, and Cl2.
Chlorides of period 3
The bonding in the chlorides of period 3 gradually changes from ionic to covalent moving from left to right. This is because the first two members of the period are the electropositive metals sodium and magnesium, which form ionic bonds. Magnesium chloride does have a degree of covalent character due to the polarising ability of the doubly charged magnesium ion.
The third member of the period is aluminium, which forms a covalent chloride. This is because the high polarising power of the small, highly charged aluminium 3+ ion prevents the chloride ions from hoding onto their electrons.
Aluminium chloride actually changes its character with temperature and it exists as a covalent dimer, Al2Cl6 at room temperature. In the gas phase the dimers dissocate to the electron deficient AlCl3 molecule.
Silicon chloride, SiCl4, is a colourless covalent liquid.
Phosphorus forms two volatile, covalent chlorides, PCl3 and PCl5, with boiling points 76ºC and 161ºC (sublimes) respectively. Phosphorus trichloride is a polar molecule (trigonal pyramidal), while phosphorus pentachloride is non-polar (trigonal bipyramidal)
|phosphorus(III) chloride||phosphorus(V) chloride|
Sulphur does not form a common chloride and is ignored in this section.
Chlorine itself is a diatomic molecular covalent non-metallic element.
Oxides of period 3
Oxygen is more electronegative than chlorine and forms ionic bonds with all metals. Hence, sodium, magnesium and aluminium have ionic oxides with very high melting points. The strength of the ionic bonding is dependent on the magnitude of the ionic charges and, to a lesser extent, on the ionic radius.
Silicon dioxide is a giant covalent macromolecule in which the whole structure is held together by strong covalent bonds. Each silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement and each oxygen atom is bonded to two silicon atoms.
Phosphorus forms two stable oxides, phosphorus(III) oxide and phosphorus(V) oxide. These are white covalent solids with melting points 24ºC and 340ºC respectively. They both have structures derived from a pyramidal arrangement of phosphorus atoms with bridging oxygen atoms.
|phosphorus(III) oxide||phosphorus(V) oxide|
Sulfur forms two oxides, sulfur(IV) oxide and sulfur(VI) oxide. Sulfur(IV) oxide (sulfur dioxide), SO2 is a covalent gas at room temperature, while sulfur(III) oxide (sulfur trioxide) is a liquid (b.p. 17ºC) at room temperature
Chlorine also forms two common oxides, chlorine(I) oxide and chlorine(VII) oxide. These are both covalent compounds.
13.1.2 - Describe the reactions of chlorine and the chlorides referred to in 13.1.1. with water.