A Short Note on What is VSEPR Theory?

VSEPR Theory helps in predicting the geometry of each molecule by the electron pair numbers which surround the central atom. Ronald Nyholm and Ronald Gillespie were the main developers of this theory therefore it is also known as Gillespie- Nyholm Theory. The basis of VSEPR Theory is that the valence electron pairs, whether bonding or non-bonding, repel each other and orient themselves in a definite geometric pattern around the central atom and the molecule acquires a definite shape.

The shared electron pairs existing between the two covalently bonded atoms is called bond pair. The other valence electron pairs of the bonded atoms not taking part in the process of bond formation are called lone pairs or non-bonding pairs of electrons. This VSEPR theory is used to find the geometry or shape of the molecule of a particular substance.

Postulates of VSEPR Theory

The important postulates of VSEPR theory are as follows;

1. The geometry and the shape of a molecule depend upon the number of valence electron pairs (whether bonding or non-bonding) present on its central atom.
2. The valence electron pairs repel one another and try to stay in space as far apart as possible to acquire a state of maximum stability.
3. The electrostatic repulsion interaction between lone pair-lone pair, lone pair-bond pair and bond pair-bond pair are different and follow the following order:

lone pair-lone pair > lone pair-bond pair > bond pair-bond pair

This electrostatic repulsion interaction between two lone pairs is maximum while that between two bond pairs is minimum because the two bond pairs lie between two atoms and are unable to undergo repulsive interactions to a larger extent. On the other hand, the lone pairs are attached to one atom only and are comparatively more free to interact. This is why a lone pair occupies more room on the surface of the central atom as compared to that occupied by a bond pair. The larger repulsion caused by the lone pairs may cause distortion in the geometry of the molecule.

4. This theory is applicable to the molecule’s resonance structures also.

Limitations of Gillespie- Nyholm Theory

The limitations of VSEPR theory are as follows;

• Gillespie- Nyholm Theory does not explain the isoelectronic species.
• This theory fails to explain the shape of transition metal compounds.
• It fails to justify the shape of group 2 halides as according to this theory they have linear shape but actually they have bent shape.

Steps for Predicting the Molecules Shapes

The steps for predicting the molecules shapes are as follows;

• Select the atom which has least or lowest electronegativity as its central atom.
• Count the number of outermost valence shell’s electron of this central atom.
• Now count the number of other atom’s electrons which are used for bonding with the central atom.
• Finally the values obtained after counting must be added to each other for obtaining VSEPR Number which will indicate the shape of the compound. 

For example; if the compound AB2 has 2 number of bond pairs then their shape will be linear as it forms the orientation of 180° around the central atom (like BeF2, BeCl2). Similarly if the central atom contains 3 valence electron pairs in their outermost shell, their mutual electrostatic repulsion interaction will compel them to acquire a trigonal planar shape with the angle of 120° between them like BF3, AlCl3 etc. In a trigonal geometry, the electron pairs are at a maximum distance from one another. Thus, the shape of any molecule can be predicted if the number of valence shell electron pairs (both bonding and non-bonding) present on the central atom is known.

Conclusion

Valence Shell Electron Pair Repulsion Theory is the VSEPR theory and is also termed as Gillespie-Nyholm Theory which helps in predicting the geometry or shape of the molecule of a compound. For example; in water (H2O) molecules, the central atom oxygen possesses 6 valence electrons. Among these, the two unpaired electrons get shared with the valence electrons of the two hydrogen atoms resulting in the formation of two O-H covalent bonds. Thus, oxygen atoms possess 2 bond pairs and 2 lone pairs. Due to electrostatic repulsion, these valence shell electron pairs orient themselves in a tetrahedral geometry around the central oxygen atom.