Question

1.Outline the postulates and limitations of VSEPR theory.

2. Explain the application of this theory to explain the shapes of the molecule namely SF4, NH3, SF6 and BCl3

Answer 1

​Postulate of VSEPR-

• the shape of the molecule is determined by the repulsion between all of the electron pairs I.e lone and bond pair present in the atom…
• A lone pair of electrons occupies more space than bond pair
• The electron pair are oriented around the central atom in such way , that the repulsion among them are minimum
• The order of repulsion among various electron pairs will be in lp-lp>lp-bp>bp-bp
• The magnitude of repulsion between bond pair's of electrons depend on the electro negativity difference between central atom and bonded atoms..
• The repulsive forces exerted by various bonds on the adjacent bond pair are in the following order Triple bond>double bond>single bond

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.