Valence Shell Electron Pair Repulsion Concept (VSEPR)

In this concept, the arrangement of bonds around the central atom is considered to depend upon the number of valence shell electron pairs, and on the relative sizes and shapes of these orbitals. These arrangements hold good for non-transition elements i.e., those elements which do not use electrons in bond formation. The geometrical shapes are actually result of the tendency of the electron-pairs to remain at a maximum distance apart so that the interaction between them is minimum. The repulsion between free electron-pairs will be obviously greater than that of repulsion between a bond pair and another bond pair. Let us sum up the essential features of this theory under the following rules:

  1. The preferred arrangement of a given number of electron pairs in the valence shell is that which makes them to remain at a maximum distance apart.
  2. A non-bonding pair (lone pair) of electrons is capable of making more space on the surface of an atom than a bonding pair. This is because the non-bonding electron pair is under the influence of one nucleus only but the bonding electron pair is constrained by two nuclei.
  3. The influence of a bonding electron pair decreases with the increasing value of electronegativity of an atom forming a molecule.
  4. The two electron pairs of a double ,bond (or the three electron pairs of a triple bond) take up more space than the one electron pair of a single bond.

Applications of Valence Shell Electron Pair Repulsion Concept:

Let us now apply the valence shell electron pair repulsion concept to explain the shapes of the molecules. In other words, the effect of electron pair repulsion on molecular structure will be discussed. The shapes of the molecules and ions of non-transition elements will now be described in terms of this theory. The molecules will be classified according to the number of electron pairs present in them, irrespective of the fact whether they are of bonding or non-bonding type.