CBSE Class 11-science Answered

Valence shell electron pair repulsion (VSEPR) rules are a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. VSEPR theory helps to predict shapes of covalent molecules.

The main postulates of this theory are as follows:

1. The shape of a molecule depends upon the number of valence shell electron pairs (bonded or non-bonded) around the central atom.
2. Electron pairs in the valence shell tend to repel each other as their electron clouds are negatively charged.
3. The valence shell is taken as a sphere with the electron pairs localizing on the spherical surface at maximum distance from one another.
4. A multiple bond is treated as a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.
5. The electron pairs tend to occupy those positions in space, which minimize repulsions and thus maximize distance from one another.
6. The VSEPR model is applicable to any such structure where two or more resonance structures can represent a molecule.

For example: In MgCl₂, there are two electron pairs and zero lone pairs. Thus, the shape of this molecule is linear.

Molecular Orbital Theory (MOT): This theory was proposedby Hund and Mulliken in 1932. The main purpose of this theory is to explain the formation of chemical bonds and to explain behaviour of molecules like their relative bond strength, paramagnetic and diamagnetic nature, stability etc.

Salient Features of Molecular Orbital Theory (MOT)

• The electrons of atoms are present in various atomic orbitals. Similarly, the electrons in a molecule are present in various molecular orbitals.
• The molecular orbitals are the energy states of a molecule, in which the electrons of the molecule are filled just like in case of atomic orbitals.
• Molecular orbitals are formed by the combination of atomic orbitals of comparable energy and proper symmetry.
• An electron in an atomic orbital is under the influence of only one nucleus while in a molecular orbital, it is under the influence of two or more nuclei depending upon the number of atoms in the molecule. Thus, an atomic orbital is mono-centric while a molecular orbital is polycentric.
• The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals combine, two molecular orbitals are formed. One is called bonding molecular orbital (B.M.O) and the other is called anti-bonding molecular orbital (A.M.O.).
• The bonding MO has greater stability than anti-bonding MO as bonding MO has lower energy than anti-bonding MO.
• A molecular orbital gives the electron probability distribution around a group of nuclei in a molecule such as an atomic orbital that gives the electron probability distribution around a nucleus in an atom.
• The, bonding molecular orbital (B.M.O) has lower energy and hence greater stability than the corresponding antibonding molecular orbital (A.M.O.).
• The B.M.Os are represented by ?, ?, ? etc. while antibonding M.Os are represented by ?^* ,?^* ,?^* etc.
• The shapes of molecular orbitals depend upon the type of combining atomic orbitals.
• The filling of M.Os takes place by same rule as for atomic orbitals like they follow, Aufbau principle, Pauli exclusion principle, and Hund's rule.