VSEPR Theory A Lewis structure shows a flat drawing of a molecule but doesn’t show the 3D shape of the molecule. In order to construct molecular shapes from Lewis structures, chemists use the valence- shell electronpair repulsion (VSEPR) theory. Basic principle: each group of valence electrons around a central atom is located as far away as possible from the others in order to minimize repulsions. An electron “group” can be bonds a single bond, double bond, triple bond, or lone pair. What we need to know to predict shape: How electrons are arranged in a molecule • • How many unshared electrons and atoms (or bonds) surround the central atom The shape is characterized by the bond angle between the central atom and the atoms bonded to it Linear: Molecular shape with two electron groups Trigonal Planar: Molecular Shapes with three electron groups Effect of Lone Pair: • • • Lone pair has an effect on the bond angle A lone pair repels bonding pairs more strongly than bonding pairs repel each other This means that lone pair electrons force the bonding pairs closer to one another, thus decreasing the bond angle Tetrahedral: Molecular shapes with four electron groups

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