IB Chemistry: Shape of Molecules and Ions

IB Chemistry: Shape of Molecules and Ions Predict the shape and bond angles for species with five and sixnegative charge centres using the VSEPR theory. VSEPR theory can be extended to cover five and six pairs of electrons ofelectrons or negative charge centers. As usual, these electron pairs will arrange themselves neatly aroundthe central atom so they are as far as away as possible to each of itsbonding partner. The shape for five negative charge centers is usually trigonal bipyramidal. The bond angles will either be 90°,120° or 180°. Six pairs will give a Octahedral Shape with bond angles of 90° and 180°. This is a molecule of Phosphorus Pentachloride , which has 5 negative charge centers.

This is a molecule of Sulfur Hexafluoride , which has 6 negative charge centers. When there are non-bonding pairs then the same rules apply, in thatnon-bonding pairs of electrons exert a greater repulsion that that ofbonding pairs. For example, consider, Xenon Tetrafluoride (XeF4) . There are six pairs of electrons around the central atom. Four are bonding pair and two arenon-bonding. This could result in two possible structures. Firstly, onepossibility is that the bond angle between the non-bonding pairs is 90°,whereas in the second structure it is 180°.