SUMMARY OF KEY CONCEPTS

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The valence-shell electron pair repulsion (VSEPR) model is used for predicting molecular shape. The primary assumption of the VSEPR model is that regions of enhanced electron density (i.e. bonding pairs, lone pairs and multiple bonds) take up positions as far apart as possible so that the repulsions between them are minimised.

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Once the basic shape of a molecule has been identified, adjustments must be made to account for the differences in electrostatic repulsion between bonding regions and lone pairs. Repulsions lie in the order: lone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair Thus for [XeF5]- the parent shape is a pentagonal bipyramid with the lone pairs opposite to each other in order to minimise lone pair/lone pair repulsions. The [XeF5]- anion is therefore pentagonal planar.

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A deficiency of the VSEPR model is that it cannot be used to predict the actual bond angle adopted by the molecule. For example, the HOH angle in water is decreased relative to that expected when all 4 electron pairs are bonding. The two lone pairs repel each other more strongly and move apart thus forcing the HOH angle to be less than the ideal tetrahedral angle of 109.5 degrees.