VALENCE SHELL ELECTRON PAIR REPULSION THEORY (VSEPR Theory) :
VSEPR theory is based on the idea that electron pairs try to be far apart as possible in the central atom of a covalent molecule. The shape of molecules depends on the number and nature of electron pairs present in the valence shell of central atom of molecule.
Postulates/Assumptions of VSEPR theory :
1.) In a covalent molecule or an ion, there is a central atom to which all other atoms are attached.
2.) Pair of electrons are present in the valence shell of central atom. They are of following types :
2.) Pair of electrons are present in the valence shell of central atom. They are of following types :
a.) Bond pair : The pairs taking part in the reaction.
b.) Lone pair : The pairs left out during bonded state.
3.) The repulsion between electron pairs are in the following order :
Bond pair and Bond pair < Bond pair and Lone pair < Lone pair and Lone pair
4.) The shape of the molecule or the ion is the function of least distance possible between the electron pairs. This reduces repulsion between the pairs.
5.) If the molecule or the ion contains only bond pairs, the molecule or the ion takes ideal geometry.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhV3Taah-GIzbyNpoPev5EXQwXJ5hJ6njV95_Nvmz9pnuAkeKCLOnXhYdL399UpBe2oX87fZkZQrq5tJiIen2uKzCjdbnlPrKxZ0k9Z8EFhC3HC5gDaNxtjXakBy1VPwK5_0a8u5ZQp0pms/s640/20170704_131148-1.jpg)
6.) If there are bond pairs as well as lone pairs in the central atom of a molecule or an ion, the ideal geometry is distorted.
7.) The shape of the molecule or the ion also depends on the electronegativity difference between central atom and other atoms.
Note :
Double/Triple bond between the central atom and other atoms like in CO2 is considered as a single electron pair according to VSEPR theory.
Learn more about VSEPR theory with examples :
- https://www.youtube.com/watch?v=GbHeKIQvcdw (Khan Academy)
- https://www.youtube.com/watch?v=nxebQZUVvTg (Tyler DeWitt)
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