VSEPR-Theory : The Postulates And Limitations

VSEPR is abbreviated as valence-shell electron-pair repulsion theory. This theory discusses  about the arrangement of atoms or group of atoms around a central atom in a covalent compound and is determined completely by the repulsion between electron pairs in the valence shell of the central atom. This theory assumes that the molecule will take a shape such that electronic repulsion in the valence shell of that atom is minimized.

VSEPR - Theory

Deciding the shape of the molecule:

  • Select the least electronegative atom as this atom will be the best to share its electrons with the other atoms in the molecule.
  • Then we count the outer shell electrons in the central atom.
  • Then we will count the electrons used by atoms (outside) to make bonds with the central atom.
  • By combining these two, we get the result as Valence shell electron pair.
  • The shape of the molecule is based on VSEP number.
VSEP Number Molecule Shape
2 Linear
3 Trigonal Planar
4 Tetrahedral
5 Trigonal Bipyramidal
6 Octahedral
7 Pentagonal Bipyramidal

Postulates of VSEPR:

  1. For polyatomic molecules containing three or more atoms, one of the atoms of that molecule is called the central atom to which all the other atoms are linked.
  2. The shape of a molecule depends upon the total number of valence shell electron pairs.
  3. The electron pairs tend to occupy positions in space such that the electronic repulsion gets minimized and hence the distance between them is maximized.
  4. The valence shell is assumed to be a sphere such that the electron pairs localize on the surface of the sphere with maximum distance between the valence shell and electron pairs.
  5. If the central atom is surrounded by bond pairs then we can expect a symmetrical shape to the molecule.
  6. If the central atom is surrounded by lone pairs and bond pairs both, then a distorted shape of the molecule is observed.
  7. In case of resonating structures, this theory can be applied to any of them.
  8. The order of repulsion between electron pairs follows the trend:
    lone pair – lone pair > lone pair – bond pair > bond pair – bond pair

Limitations of VSEPR theory:

  • VSEPR Theory fails to explain isoelectronic species. Isoelectronic species refers to the elements that have same number of electrons. According to VSEPR theory, a shape of the molecule depends on the number of bond pair and a lone pair (valence electrons).But, isoelectronic species can differ in the shape even if they have a same number of electrons.
  • VSEPR Theory does not explain the transition metal compounds. The VSEPR model fails to guess the structure of certain compounds. The reason being that it does not take associated sizes of the substituent’s and inactive lone pairs into account.

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Practise This Question

Which among the following compound have non-zero dipole moment?