In molecular orbital theory, atomic orbitals combine to form molecular orbitals. Here, we will discuss the two types of MO formed due to LCAO method.

What is molecular orbital theory

1. In VBT, half filled atomic orbitals overlaps to form a covalent bond. But these atomic orbitals do not lose their identity.
2. Atomic orbitals combine to form new set of MOs in MOT. And atomic orbitals lose their identity in the process.
3. Only those atomic orbitals combine which are of comparable energy and proper orientation.
4. Number of MO formed are equal to number of atomic orbitals combined.
5. Two atomic orbitals combine to form 2 types of MOs
6. One MO forms with lower energy is bonding MO and is more stable.
7. The other MO forms with higher energy is antibonding MO and is less stable.
8. The filling of electrons takes place according to Aufbau Principle, Pauli’s Principle and Hund’s rule of multiplicity.
9. Bonding MO are of 2 types: sigma and pi MO
10. Antibonding MO are also of 2 types: sigma*, Pi*

Rules for filling electronic configuration of MO

1. Aufbau’s Principle

The rule states that electrons are filled in order of increasing energy.

2. Pauli’s Exclusion Principle

A MO can have maximum of 2 electrons with opposite spins.

3. Hund’s Rule of maximum multiplicity

Pairing in an orbital will not start until all the degenerate orbitals in a given sub-shell is singly occupied.

Difference between atomic orbitals and MO

1. Atomic orbitals are less stable. MO are more stable.
2. AO have simple shapes. MO have complex shapes.
3. Atomic orbitals are represented by s, p, d and f. MO are represented by sigma, sigma*, Pi, Pi* means bonding & antibonding MO
4. AO is monocentric while MO is polycentric.

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