Question

what is vesper theory

Answer 1

Valence Shell Electron pair repulsion theory is a model used in chemistry to predict the geometry of individual molecules from the number of Electron pairs surrounding their cental atoms. It is also named as Gillespie Nyholm theory after its two main developers- Ronald Gillespie and Ronald Nyholm...

Answer 2

$\huge\textbf{VESPER theory}$

[Valence Shell Electron Pair Repulsion Theory]

→ Lewis concept is unable to explain the shape of molecules. Then VESPER Theory explain the shapes of molecules. 

→ According to this theory, "The electron pair surroundimg the central atom repel one another. So, they moves away such that the repulsion between them and molecule has minimum energy and maximum stability.
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$\textbf{Postulates of VESPER theory}$

1) Shape of molecules depends upon the number of valence shell electron pairs around the central atom.

2) Valence shell electron pair repel each other so they take position such that they experience minimum repulsion.

3) Valence shell electrons which participate in bonding are called Bonding pair and which don't take part in bonding are called Non-bonding electron or lone pair.

* Their order of repulsion is LP - LP › LP - BP › BP - BP.

4) When molecule has only bond pair, then molecules has regular geometry.

5) When molecule or central atom has bond pair as well as lone pair, then it has irregular geometry.

→ Based on above postulates, molecules has various geometry and shape like linear, see-saw, T- shape, Tetrahedral, trigonal pyramidal, trigonal bipyramidal, octahedral, square planner, square bipyramidal. 
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$\textbf{Limitations}$

→ Lewis and VESPER has many limitations to explain all the molecules shape. These limitations are explained by 2 Modern Theories, which are based upon quantum mechanics. 

→ These 2 Modern Theories are :-

* Valence bond theory.

* Molecular orbital theory.
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