Ch 2: Bonding

Chapter 2 : Alkanes

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 anti-bonding molecular orbitals.

We can use the same basic diagram that we used for hydrogen since the same type of atomic orbitals, the 1s orbitals, are involved. The only difference is that they now contain a pair of electrons.

Everything is the same until we put 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 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. So 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.