VSEPR Theory

VSEPR Theory
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VSEPR Theory ( Valence Shell Electron Pair Repulsion Theory)-

Sidgwick and Powell proposed a theory based on the repulsive interactions of electron pairs of valence shell.This theory was developed by Nyholm and Gillespie.

                The basis of this theory is the shape of molecules can be determined by the number of lone pair and bond pair of electrons in valence shell of central atom.

Postulates of VSEPR Theory (Valence Shell Electron Pair Repulsion Theory)-

1) The geometry of a covalent molecule depends upon the number of bp of electrons and number of lp of electrons .

Hybridisation = no. of lone pair(lp) of electrons + no.  of bond pair (bp)of electrons

Ex . CH₄

bp =4 , lp =0

Hybridisation =  0 + 4 =4  [sp³ hybridisation]

Geometry tetrahedral and bond angle is 109⁰28′

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NH₃

bp =3 , lp =1

Hybridisation =  1 + 3 =4  [sp³ hybridisation]

Geometry  pyramidal  and bond angle is 106⁰45′

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2) Electron pairs tend to repel one another because  their electron clouds are negatively charged.

3) If all the hybrid orbitals of central atom contain only bond pair of electrons then the molecules will have regular geometry.

4). If  hybrid orbitals of central atom contain lone pair and bond pair of electrons then the molecules will have irregular geometry.

Ex.

H₂O

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SF4

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5) The electron pair surrounding the central atom repel each other. This implies that orbitals or electron clouds containing electron pairs repel each other. Therefore they try to stay as far apart as possible to acquire a state of minimum energy and maximum stability.

6) The repulsive interactions between two lone pairs are different from those between two bps or those between a lone pair and bond pair . The repulsive interaction between lone pair and bond pair decreases as –

lp – lp > lp – bp > bp – bp