Name: Hybridisation in SF₆ Molecule
Uploaded: 2019-07-19
Duration: 2 min 31 s
Description: Understand the bonding present in the case of sulphur hexafluoride. This video illustrates that the hybridisation present in the case of sulphur hexafluoride...

Question

Give an account of VSEPR theory and its applications.

Accepted Answer

In order to explain the shapes of the molecules adequately, Sidgwick and Powell in $1940$ proposed a valence shell electron pair repulsion theory based on the repulsive interaction of the electron pairs in the valence shell of the atoms. Nyholm and Gillespie further developed and redefined the concept. The main postulates of this theory are as follows:

(i) The number of valence shell electron pairs (bonded or non-bonded) present around the central atom decides the shape of the molecules. (The shared electron pairs are called bond pairs and unshared or non-bonding electrons are called lone pairs).

(ii) Electron pairs of valence shell repel one another because their electron clouds are negatively charged.

(iii) These electron pairs arrange themselves in such a way so that there is minimum repulsion and maximum distance in between them.

(iv) The valence shell is considered as a sphere in which the electron pairs are localised on the spherical surface at maximum distance from one another.

(v) A multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.

(vi) If a molecule has several resonating structures, the VSEPR model is applicable to any such structure.

(vii) A lone pair occupies more space than a bonding pair, since it lies closer to the central atom.
This means that the repulsion between the different electron pairs follow the order:
Lone pair-lone pair $>$ lone pair-bond pair $>$ bond pair — bond pair {(lp-lp) $>$ (lp-bp) $>$ (bp-bp)}.

(viii) Repulsive forces decrease sharply as the angle between the electron pairs increases. They are strong at $90^{°}$ , weaker at $120^{°}$ and weakest at $180^{°}$ .

Application of VSEPR theory:

Valence shell electron pair repulsion (VSEPR) theory is used in chemistry to predict the three-dimensional shapes of molecules from the number of electron pairs surrounding their central atoms. Molecules can have linear, trigonal planar, bent, tetrahedral, trigonal bipyramidal, see-saw, T-shaped, octahedral, square pyramidal or pentagonal bipyramidal geometries.

Give an account of VSEPR theory and its applications.

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