Ch 2: Bonding

Chapter 2: Bonding

Bonding in "He₂"
Why can't helium form bimolecular helium molecules (He₂) analogous to H₂ ?

Answering this question provides insight into the relevance and importance of the anti-bonding molecular orbitals.

Good news ! We can use the same basic molecular orbital diagram that we used for hydrogen because the starting point is the same set of atomic orbitals, the 1s orbitals. The only difference is that the number of electrons involved. Each 1s orbital now contains a pair of electrons.

Everything is the same until we consider in the electrons.

Now we have two electrons in the in-phase combination, the bonding molecular orbital. But we also have two electrons in the out-of-phase combination, the anti-bonding molecular orbital

The stabilisation due to the electrons in the σ-bonding orbital is given by 2 DE.

The destabilisation due to the electrons in the σ^*-anti-bonding orbital is give by 2 DE^*.

The critical factor from the quantum mechanical treatment is that the magnitude of DE^* > DE.

Hence, the destabilisation due to the electrons in the σ^*-anti-bonding orbital is greater than the stabilisation due to the electrons in the σ-bonding orbital : the net effect being there is more destabilisation than there is stabilisation. Therefore, a molecule of He₂ is less stable than 2 atoms of He !

molecular orbitals for the diatomic helium, He2

He
atomic
orbital

He₂
molecular
orbital

He
atomic
orbital

Conclusion

The presence of electrons in the σ^*-anti-bonding orbital prevents He₂ from forming, hence it cancels out the effect of the electrons in the σ-bonding orbital.