Molecular geometry is the three-dimensional arrangement of atoms in a molecule. A molecule’s geometry affects its physical and chemical properties, such as melting point, boiling point, density, and the types of reactions it undergoes. In general, bond lengths and bond angles must be determined by experiment. However, there is a simple procedure that enables us to predict with considerable success the overall geometry of a molecule or ion if we know the number of electrons surrounding a central atom in its Lewis structure. The basis of this approach is the assumption that electron pairs in the valence shell of an atom repel one another.

The basic principle of VSEPR theory is that the pair of valence shell electrons around the central atom stays as far apart from each other as possible to minimize repulsion among them. For simplicity, a set of electrons is defined as any number of electronpairs that occupies a localized region around a central atom. This set may consists of a single bond (–), a double bond (=), a triple bond (≡), lone-pair(s) or in some cases even a lone (single) electron. The three-dimensional arrangement of these sets gives rise to the shape of the molecule.

Electron Pair Arrangement and Molecular geometry

The electron pair arrangement is defined by the sets of electrons, both bonding and non-bonding (lone-pair), around the central atom. On the other hand the molecular shape is defined by the relative positions of the atomic nuclei. To classify molecular shapes, the AX_mE_n designation is assigned, where A is the central atom, X is the terminal atom, E is lone-pair (nonbonding) electron sets, m and n are positive integers.

The arrangements that best minimize repulsions naturally depend on the number of electron sets. Repulsive forces among valence pairs diminish in the following order:

Lone pair - lone pair > lone pair - bond pair > bond pair - bond pair.

In this section, you will see electrons set ranging from two to six around the central atom. Two electron sets locate themselves on opposite sides of the atom in a linear arrangement, three sets form a trigonal planar structure, four sets arrange themselves at the corners of a tetrahedron, five sets define a trigonal bipyramid, and six sets form an octahedron.

Guidelines for Applying VSEPR Model

Here are some guidelines for applying the VSEPR model:

1. First, write the Lewis structure of the molecule. In this model, consider only the electron sets (pairs) of the central atom.
2. Count the total number of electron sets around the central atom, including both the bonding pairs and lone pairs. Usually a lone pair, a single unpaired electron and any bond: single, double or triple, each count as one area of electron density.
3. Use the VSEPR geometry to predict the shape of the molecule.

In explaining why the distortion occurs you need to balance three competing influences:

• Lone pairs take up more space than bonding pairs. Repulsions between electrons in a lone-pair and the other electrons (in either bonds or other lone pairs) may cause distortions in the structure.
• The amount of electron-electron repulsion experienced between a bond and lone pairs or other bonding pairs decreases in the order:

triple > double > single

• Therefore, we expect that a triple bond may cause more distortions in the structure than either a double or single bond and that a double bond will cause more distortion when compared to a single bond.
• Therefore, we expect that a triple bond may cause more distortions in the structure than either a double or single bond and that a double bond will cause more distortion when compared to a single bond.