Formation of molecular orbital occur by the linear combination of atomic orbitals. Here we will discuss about the types of MO and their formation.

What is LCAO

1. An atomic orbital is an electron wave.
2. Constructive interference involves the 2 waves which are in phase. They add up to form the amplitude of the new wave. Bonding MO formed by the additive effect of atomic orbitals.
3. Destructive interference involves the 2 waves which are out of phase. Subtraction of the waves form the amplitude of the new wave. Antibonding MO formed by the subtractive effect of atomic orbitals.
4. Crest of a wave is a + wave sign. Trough of a wave is a – sign.
5. Bonding MO forms by the combination of + & + sign and – & – sign
6. Antibonding MO forms by the overlap of + & – sign

Difference between bonding and antibonding molecular orbital

1. Bonding MO forms by the additive effect of the atomic orbitals. Antibonding MO forms by the subtractitive effect of the atomic orbitals.
2. The electron density increases in the region between the nuclei of bonded atoms in bonding MO. While electron density decreases in the region between the nuclei of the atoms in antibonding MO
3. Bonding MO formed have lower energy than the atomic orbitals involved in combination due to the attractive forces. This results in more stability than the antibonding MO which are less stable.
4. In the bonding MO, the lobes of the combining orbitals have the same sign. But in the antibonding MO, the lobes of the combining orbitals have the different sign.
5. Bonding MOs are of 2 types – sigma and pi. But antibonding MOs are also of 2 types – sigma* & Pi*

Conditions for the combination of atomic orbitals to form MO

1. The combining atomic orbitals should have comparable energies. For example: 1s combines with 1s. And 2s can combine with 2s but not with 2p orbital.
2. They should have proper orientation for the efficient overlapping. For example 2pz can combine with 2pz orbital but not with 2px or 2py orbitals.
3. The extent of overlapping should be large.

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