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central atom and the molecular shape of the ion ClO3–.

Applying VSEPR theory to simple molecules

The methane molecule, CH4, can be used to illustrate the procedure for predicting molecular shape. The Lewis structure of this molecule ascribes four bonding electron pairs to the carbon atom (Figure 8). These pairs repel one another, and their separation is maximized if they adopt a tetrahedral disposition around the central carbon atom. A hydrogen atom is attached by each bonding pair, so it can be predicted that CH4 is likely to be a tetrahedral species, which is in fact the case.methaneFigure 8: The structure of methane, CH4. This regular tetrahedral structure is explained in the VSEPR theory of molecular shape by supposing that the four pairs of bonding electrons (represented by the gray clouds) adopt positions that minimize their mutual repulsion.Encyclopædia Britannica, Inc.Advertisement

When applying VSEPR theory, attention is first focused on the electron pairs of the central atom, disregarding the distinction between bonding pairs and lone pairs. These pairs are then allowed to move around the central atom (at a constant distance) and to take up positions that maximize their mutual separations. As in the methane molecule, four pairs adopt a tetrahedral disposition. The arrangements adopted by two through six pairs are summarized in the table. At this stage, the atoms that are attached by the bonding pairs are introduced, and the shape of the molecule is reported on the basis of the arrangement of these atoms.

The water molecule, H2O, provides a simple example. The oxygen atom has four electron pairs, so these pairs adopt a tetrahedral arrangement. Two of the pairs are bonding, and hydrogen atoms are attached to them. Hence, the molecule is angular. (Note that the shape of the molecule is determined by the disposition of the atoms, not the disposition of the electron pairs.) The ammonia molecule, NH3, has four electron pairs in a tetrahedral arrangement around the nitrogen atom; three of these pairs are used to bond hydrogen atoms, so the molecule is predicted to be trigonal pyramidal, with a lone pair in the apical position. Some of the names of the shapes of simple molecules are summarized in the table.