ISC Chemistry - Grade 11- Chemical Bonding - VSEPR Theory

VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY

Valence Shell Electron Pair Repulsion (VSEPR) theory enables us to understand why molecules have certain characteristic shapes. The main points of this theory are:

1.     In a polyatomic molecule the orientation (direction) of the bonds, around the central atom depends upon the total number of electron pairs (bonding as well as non–bonding) in its valence shell.

2.     There is mutual repulsion between these electron pairs. Consequently, they stay as far away as possible from each other to reduce the repulsion and to attain maximum stability.

3.     The force of repulsion between bonding pairs and non–bonding pairs are different. [Non–bonding pair of electrons is also known as a Lone Pair.] 

The decreasing order of repulsion between the two types of electron pairs is given below:

[Lone pair – lone pair] > [lone pair-bonding pair] > [ bonding pair – bonding pair ]

Further, it can be said that,

The molecule will have a regular geometric shape if all the repulsive interactions between the electron pairs around the central atom are equal. The molecule will have an irregular or distorted geometric shape if the repulsive interactions are unequal. Repulsive forces decrease sharply with an increasing angle between the electron pairs. They are strong at 90°, weak at 120° and weakest at 180°. The final geometry or the shape of the molecule is the net result of balancing of all interactions between the various bonds existing in the molecule to give a stable structure.

The shape of BF₃ molecule

Boron atom B (1s², 2s²  2p¹) is the central atom and it has three valence electrons. These three electrons form three B–F covalent bonds with three F atoms. The central B atom is surrounded by three bond pairs of electrons which will acquire trigonal planar geometry. Thus, BF₃ molecule has a trigonal planar shape as shown in

Geometry of Boran TriFloride

 The shape of a methane CH₄ molecule

Carbon atom C (1s²,  2s²,  2p²) is the central atom of the methane molecule. C atom has four valence electrons which are shared mutually with four hydrogen atoms to form four C––H covalent bonds. The four bond pairs of electrons acquire a tetrahedral geometry. Therefore, CH₄ molecule has tetrahedral structure as shown in the representation 

Geometry of Methane

The shape of NH₃ molecule

In ammonia molecule, N (1s², 2s² , 2p³) atom is the central atom to which three hydrogen atoms are bound by three N–H covalent bonds. So, out of the five electrons in the valence shell, one pair of electrons forms the non–bonding or the lone pair of electrons.
Thus, there are four electron pairs (three bond pairs and one lone pair) around the central nitrogen atom. These four electron pairs give a tetrahedral structure (just like the one in CH₄). But the presence of a lone pair causes distortion. There is greater repulsion between the lone pair and the bonding pairs and as a result, the bonding pairs move closer to each other than in a regular tetrahedron arrangement. Therefore, the ammonia molecule has a distorted tetrahedral geometry. The bond angle H–N–H decreases from 109.5° to just 107°.
The geometry of ammonia molecule is also regarded as pyramidal, as shown in the figure:

Geometry of Ammonia 

Memory  Map for VSEPR Theory

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