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Molecular Shapes and VSEPR Theory

Valence shell electron pair repulsion theory (VSEPR) takes a different view from that of the non-bonded radii approach for the adsorption of a given geometry as is possible within a non-quantum framework. Gillespie and Nyholm developed the theory soon after Lewis published his concept of the arrangement of electrons in pairs in a molecule.

Valence shell electron pair repulsion theory (VSEPR) is a simple powerful method for predicting the molecular shapes of main group species. While VSEPR can be used to predict shapes, it says nothing about the bonding in main group compounds. VSEPR theory helps us to select the kind of hybrid orbitals an atom involved in the formation of chemical bonding. VSEPR theory also predicts the orientation of electron groups around an atom.

Basic Postulates of VSEPR Theory

The basic postulates of VSEPR theory are

  1. A molecule’s shape relies on the amount of pairs of valence shell electron around the core atom.
  2. Electron pairs repel each other in the valence shell as their electron clouds are charged negatively.
  3. These pairs of electrons tend to occupy such space positions that minimize repulsion and thus maximize their distance.
  4. The valence shell is chosen as a sphere with the pairs of electrons located on the spherical surface at peak distance from each other.
  5. A multiple bond is regarded as a single couple of electrons, and a multiple bond has two or three electron pairs which are treated as a single super couple.
  6. The VSEPR theory applies to any such structure when two or more resonating structures can constitute a molecule.

Does VSEPR Theory account for Bond Strength?

Yes, VSEPR theory explains the geometry the molecular geometry based on the total number of bonded or non-bonded electrons around the central atom in the molecule. Repulsion between valence shell electron pairs determines molecular shape. Repulsion between a lone pair of electrons is different from that between the bond pairs or one lone pair and one bond pair. The repulsive interactions decrease in the order Lone pair-Lone pair > Lone pair-Bond pair > Bond pair-Bond pair.

The presence of lone pairs in addition to bond pairs will result in certain distortions in the regular geometry of molecules. Thus the molecules in which the central atom is surrounded only by similar bonded electron pairs will have regular geometries while those in which the central atom is surrounded by bond pairs as well as lone pairs will have irregular geometries.