CBSE Class 12-science Answered

 Molecular orbital theory

1.  Molecular orbital theory asserts that atomic orbitals no longer hold significant meaning after atoms form molecules. Molecular orbital theory holds electrons reside in molecular orbitals that are distributed over the entire molecule.
2.  Atomic orbitals of comparable energy and proper symmetry combine together to form molecular orbitals. The total number of orbitals is conserved; the number of   molecular orbitals equals the number of original atomic orbitals.
3.  When atomic orbitals interact, if added them it results in a bonding orbital and if subtracted then an antibonding orbital is obtained. Antibonding orbitals are denoted with an asterisk (*).
4.  The movement of electrons in a molecular orbital is influenced by all the nuclei of combining atoms.

Valence bond theory:

1. It does not give quantitative interpretation of magnetic data.
2.  It does not explain the colour exhibited by coordination compounds.
3.  It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.
4.  It does not make exact predictions regarding the tetrahedral and square planar structures of 4-coordinate complexes.
5.  It does not distinguish between weak and strong ligands.

According to this theory, the metal atom or ion under the influence of

ligands can use its (n − 1)d, ns, np or ns, np or nd orbitals for hybridisation

to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral and square planar.

These hybridised orbitals are allowed to overlap with ligand orbitals which can donate electron pairs for bonding.