James Richard Fromm
Although the prediction of molecular geometry is most correctly done by quantum-mechanical calculations involving the orbitals on the different atoms which make up the compound, this approach is beyond the scope of the present section. However, it is quite possible to predict the geometrical structures of many compounds without calculations by using what has come to be called valence shell electron-pair repulsion, abbreviated as V.S.E.P.R.
The fundamental principle of valence shell electron-pair repulsion is that when a molecule consists of a central atom which is surrounded by other atoms, the geometry of the molecule will be such that all of the electron pairs in the valence shell of the central atom will be as far from each other as possible. This will be true whether the electron pairs in the valence shell are used for bonding or not. Thus if a central atom has two electron pairs in its valence shell, the basic structure of the molecule is linear; if there are three, it is triangular; if there are four, it is tetrahedral; if there are five, it is a trigonal bipyramid; and if there are six, it is an octahedron.
Once the basic shape of a molecule has been established, and in this regard the simple V.S.E.P.R. approach gives the same geometric pattern as does the more sophisticated hybridization of atomic orbitals approach, the substituents can be arranged in the basic shape. For the two larger basic shapes, five and six electron pairs in the valence shell, the possible arrangements of substituents (noncentral atoms) and nonbonding electron pairs is not unique. In arranging the substituents and electron pairs about the central atom of a trigonal bipyramid, the unshared electron pairs should be placed in equatorial positions. In an octahedron, the unshared electron pairs should be placed as far from one another as possible. For example, if an octahedral basic structure contains four substituents and two unshared electron pairs, the two unshared pairs should not be placed adjacent to each other.
The electron pairs take up positions as far from each other as possible. The repulsion can easily be observed in a physical model simply by tying balloons together tightly in the appropriate numbers. Four identical balloons will form a tetrahedron while six identical balloons will form an octahedron.
The number of electron pairs in the valence shell establishes the basic shape of a molecule. Since the number of electron pairs used in the valence shell of the central atom is shown in the Lewis structure of a molecule as discussed earlier in a preceding section, the basic shape of many molecules can be deduced directly from their Lewis structures.