EXPLAIN VSEPR THEORY WITH EXAMPLE

VSEPR is nothing but the repulsion between valence electron pairs of atoms and this is used to predict the shape and geometry of atoms. VSEPR models are primarily based on the idea that electrons revolving around a core atom will arrange themselves to reduce repulsion, dictating the molecule’s shape.

VSEPR theory:

Valence Shell Electron-Pair Repulsion Theory is also known as VSEPR.

The theory talks about the repulsion between valence electrons pairs of atoms. It means it is trying to say that all atoms have valence electrons pair and they repel, the repulsion is nothing but the VSEPR (Valence Shell Electron Pair Repulsion Theory). As a result of repulsion, the atoms try to arrange themselves so that the repulsion is minimum. It is nothing but a helpful structure or model for the prediction of geometry and shapes. It is very helpful to approximately all compounds having a nonmetal central atom. 

VSEP number:

VSEP is the number given to a particular shape of the molecule and describes a particular shape of a molecule. There are some basic shapes of molecules that are used in VSEPR (Valence shell electron pair repulsion theory). The basic shapes example chart is: 

 The shape of molecules:

There are some basic shapes of molecules that are used in VSEPR (Valence shell electron pair repulsion theory). The basic shapes are :

1. Linear shape:

In linear shape, there are only 2 places on the central atom valence shell and the arrangement between them is such that the repulsion between them is minimum or can be minimized.

Example: BeF2

 Trigonal planar shape:

In Trigonal planar shape, there are 3 places on the central atom valence shell and the arrangement between them is such that the repulsion between them is minimum or can be minimized.

Example: BF3 

3. Tetrahedral shape:

Atoms lie on the same plane if we put them on a 2-dimensional plane and if we apply these conditions in CH4 that is methane, then a square planar shape we will get and the bond angle between them is 90 degrees.

Atoms lie on the same plane if we put them on a 3-dimensional plane and if we apply these conditions in CH4 that is methane, then a tetrahedral shape we will get and the bond angle between them is 109 degrees and 28’.

 Trigonal bipyramid shape:

           Along the equator of the molecule, three positions lie in a trigonal bipyramid. 

Example: PF5

 Predict the shapes of molecules:

To determine the geometry of a molecule, the procedures below must be performed.

• The smallest electron-deficient atom should always be chosen as the center atom.

• It is necessary to count the whole number of electrons in the core atom’s outermost shell.

• It is necessary to count the whole number of electrons from other atoms that are employed in bonding with the central atom.

• To get the VSEP number, combine these two numbers.

Importance:

• Because Lewis structures are confined to two dimensions, they can only tell you how much and what kinds of links exist between atoms. The VSEPR model accurately predicts its three-dimensional structure of molecules and ions, but it fails to provide any detailed information on link length or bond structure.

• VSEPR models are primarily based on the idea that electrons revolving around a core atom will arrange themselves to reduce repulsion, dictating the molecule’s shape.

• As provided since the central atom would not be metal, this could predict the form of practically all compounds with a central atom. Each form is given a name as well as an imagined bond angle.

Limitations:

• Isoelectronic species are not explained by this idea. Despite it having the same amount of electrons, the morphologies of the species might differ.

• The VSEPR theory provides little insight into transition metal compounds. This hypothesis cannot accurately describe the structure of numerous such molecules. This is due to the fact that the VSEPR hypothesis ignores the substituent groups’ associated sizes as well as the inactive lone pairs.

• Another flaw in VSEPR theory is whether it predicts that group 2 element halides would have a regular pattern when they really have a bent structure.

VSEPR example problems:

Q1) Describe the structure of the NH3 molecule using the VSEPR theory.

ANS: “N” is the most central atom in ammonia. Because “N” is a group of 15 elements, it contains 5 electrons in the outer shell. The lone pair is formed by the remaining two electrons of N that are not involved in the hydrogen bonding. The outer shell eventually receives eight electrons, three pairs of linked electrons and one lone pair. 

These 4 electron pairs form a tetrahedral structure, with H atoms occupying three spots and the lone pair occupying the fourth. This form can be classified as either tetrahedral or pyramidal. From 109°28′ to 110°28′, the existence of a lone pair generates a small distortion.

 Conclusion:

VSEPR is nothing but a helpful structure or model for the prediction of geometry and shapes. It is very helpful to approximately all compounds having a nonmetal central atom. So we use VSEPR as a geometrical shape or structure maker.