Molecular Orbital Treatment of Homonuclear Diatomic Molecules H₂ and He₂

How to Construct Molecular Orbital Diagram Simplest Trick

Hydrogen (H₂) molecule:

A hydrogen atom has electronic configuration 1s¹.

1s atomic orbitals of two hydrogen atoms combine to form two molecular orbitals, the bonding molecular orbital σ1s and antibonding molecular orbital σ*1s. The electron filling in these molecular orbitals follows Aufbau and Pauli exclusion principle. Both electrons go to bonding molecular orbital and not in the antibonding, so a strong bond will be formed.

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How to draw the Molecular Orbital Diagram of H2 ( Hydrogen molecule ) ?

Bond order = Number of electrons in BMO – Number of electrons in ABMO

= 2 – 0 = 1

As bond order in H₂ is one so one bond i.e a single bond is formed between two hydrogen atoms. Furthermore, as there is no unpaired electron in the H₂ molecule hence it is diamagnetic.

Helium molecule He₂:

Helium atom has electronic configuration 1s².

1s atomic orbitals of two helium atoms combine to form two molecular orbitals, the bonding molecular orbital σ1s and antibonding molecular orbital σ*1s.

The electron filling in these molecular orbitals follows Aufbau and Pauli exclusion principle. Out of four electrons, two electrons go to the bonding molecular orbital and two to the antibonding molecular orbital, so no bond will be formed.

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How to draw the Molecular Orbital Diagram of He₂ ( Helium molecule ) ?

Bond order = Number of electrons in BMO – Number of electrons in ABMO

= 2 – 2 = 0

As bond order in Helium is zero so no bond is formed between two helium atoms. Furthermore, as there is no unpaired electron in the Helium molecule hence it is diamagnetic.