Question

Write the postulate of VESPR theory???

Answer 1

hi ✌️

VESPER THEORY
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( valence shell electron pair repulsion theory)

This is a simplified theory to predict the shape of simple molecule .The method was originally proposed by sidwick and Powell in 1940 .

It was further developed and relined by Nyholm and Gillespie.

☑️ The postulates of this theory are:-

⚫ The shape of the molecules is determined by repulsion between all of the electron pairs present in the valence shell of the central atom.

⚫ The electron pairs tend to occupy position in space that minimize repulsion.Therefore ,they try to stay as far as possible to acquire a state of minimum energy or maximum stability.

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

⚫ The electron pair in a multiple bond are treated as a single unit like a single electron pair.

⚫ The shared pairs of electrons are called bond pair and unshared pairs of electrons on the central atom are called lone pairs.

⚫ The magnitude of repulsion force between different electron pair is not equal.

The decreasing order of repulsion force is;

L.P-L.P > L.P-B.P > B.P-B.P

A loan pair takes up more space than a bond pair and due to this repulsion force between the loan pair is maximum.

⚫ If the central atom of a molecule is sorrounded by bonding electron pair only, then the molecule have a regular geometry.

⚫ When the central atom of a molecule is sorrounded by both loan pair and bond pair , then the molecule doesn't have a regular geometry. The molecule have a irregular geometry.

⚫ Bond angles involving multiple bonds are generally larger than those involving only single bond. However the multiple bonds do not effect the geometry of a molecule.

⚫ Repulsion between electron pairs in filled shells are larger than repulsion between electron pair in incompletely filled shells.

⚫ When the central atom is attached with dissimilar atom, the molecule have a irregular geometry.


(Geometry of various molecules....see the above picture)

hope it will help you ☺️

Answer 2

$\textbf{VESPER theory}$

[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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$\huge\textbf{Postulates of VESPER theory}$

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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