Chemistry VSEPR Theory

Topics Covered :

● VSEPR Theory
● Postulates of VSEPR Theory
● Refinement of VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory :

`=>` Lewis concept is unable to explain the shapes of molecules.

`=>` This theory provides a simple procedure to predict the shapes of covalent molecules.

`=>` Sidgwick and Powell in 1940, proposed a simple theory based on the repulsive interactions of the electron pairs in the valence shell of the atoms.

`=>` It was further developed and redefined by Nyholm and Gillespie (1957).

The main postulates of VSEPR theory are as follows :

● The shape of a molecule depends upon the number of valence shell electron pairs (bonded or nonbonded) around the central atom.

● Pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged.

● These pairs of electrons tend to occupy such positions in space that minimise repulsion and thus maximise distance between them.

● The valence shell is taken as a sphere with the electron pairs localising on the spherical surface at maximum distance from one another.

● 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.

● Where two or more resonance structures can represent a molecule, the VSEPR model is applicable to any such structure.

● The repulsive interaction of electron pairs decrease in the order :

`color{red}("Lone pair (lp) – Lone pair (lp) > Lone pair (lp) – Bond pair (bp) > Bond pair (bp) – Bond pair (bp)")`

Refinement of VSEPR Theory :

`=>` Nyholm and Gillespie (1957) refined the VSEPR model by explaining the important difference between the lone pairs and bonding pairs of electrons.

● While the lone pairs are localised on the central atom, each bonded pair is shared between two atoms.

● As a result, the lone pair of electrons in a molecule occupy more space as compared to the bonding pairs of electrons.

● This results in greater repulsion between lone pairs of electrons as compared to the lone pair - bond pair and bond pair - bond pair repulsions.

● These repulsion effects result in deviations from idealised shapes and alterations in bond angles in molecules.

`=>` For the prediction of geometrical shapes of molecules with the help of VSEPR theory, it is convenient to divide molecules into two categories as :

(i) molecules in which the central atom has no lone pair and (ii) molecules in which the central atom has one or more lone pairs.

`=>` Table shows the arrangement of electron pairs about a central atom `A` (without any lone pairs) and geometries of some molecules/ions of the type `AB`.

`=>` Table shows shapes of some simple molecules and ions in which the central atom has one or more lone pairs.

`=>` Table explains the reasons for the distortions in the geometry of the molecule.


`=>` As depicted in Table in the compounds of `AB_2`, `AB_3`, `AB_4`, `AB_5` and `AB_6`, the arrangement of electron pairs and the `B` atoms around the central atom `A` are : linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral, respectively.

● Such arrangement can be seen in the molecules like `BF_3 (AB_3), CH_4 (AB_4)` and `PCl_5 (AB_5)` as depicted below by their ball and stick models.

`=>` The VSEPR Theory is able to predict geometry of a large number of molecules, especially the compounds of `p`-block elements accurately.

`=>` It is also quite successful in determining the geometry quite-accurately even when the energy difference between possible structures is very small.

`color{green}("Disadvantage :")` The theoretical basis of the VSEPR theory regarding the effects of electron pair repulsions on molecular shapes is not clear.



 
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