UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper I

Q1(f). Existence of hydrogen and helium dimers

Question

Explain why H₂ molecule exists but He₂ does not.

H₂ has bond order 1, whereas the simple ground-state MO configuration of He₂ has bond order 0.

In H₂, the two electrons occupy bonding σ₁ₛ: (σ₁ₛ)². Hence BO=(2-0)/2=1 and bond formation lowers the energy.
In He₂, four electrons give (σ₁ₛ)²(σ*₁ₛ)². Bonding and antibonding contributions cancel: BO=(2-2)/2=0.
Therefore a normal covalently bound ground-state helium molecule is not predicted. This textbook statement excludes the extremely weak van der Waals helium dimer.

The full model answer

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CoversExistence of hydrogenhelium dimersMolecular orbital theory