21 Chemistry -- Chemical Bonding and Shapes of Molecules

State pauli exclusion principle. Describe valence bond theory with its postulates.

State pauli exclusion principle. Describe valence bond theory with its postulates.

Wolfgang Pauli in 1925, put forward a principle that restrict some of the values of quantum numbers of electrons in an atom. Pauli's exclusion principle states," no two electrons in an atom can all the four quantum numbers same." In other word if three of the quantum numbers for two electrons in an atom are identical, the fourth quantum number for the two electrons should differ. 

Heitler and London in 1927 developed valence bond theory based on quantum mechanical model of an atom. According to this theory, covalent bond is formed by overlapping of atomic orbitals of the outermost shell containing the electrons of opposite spins. Postulates of valence bond theory are:

1. Covalent bond is formed by overlapping of half- filled atomic orbitals of valence shells of the combining atoms with the electrons having opposite spin.

2. The overlapping of two atomic orbitals gives rise to a single bond orbital which is occupied by two electrons from the combining atoms.

3. Pairing of electrons in overlapping region releases energy. This is the cause of stability of the molecule because the energy of the molecule formed become less than the combining atoms.

4. The extent of overlapping determines the strength of bond i.e. the higher the extent of overlapping, the greater the strength of bond.

5. The nature of bond depends upon the orbitals involved in bonding. For example, the bond formed by non-spherical orbital is stronger than that formed by spherical orbital 


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