JEE Main · 2018 · 08 Apr 2018 · Offline · Chemical Bonding and Molecular Structure

According to molecular orbital theory, which of the following will not be a viable molecule?

(A) $H_{2}^{2 -}$ ✓ Correct
(B) $He_{2}^{2 +}$
(C) $He_{2}^{+}$
(D) $H_{2}^{-}$

Answer: (A)

Solution

Molecular orbital theory fixes stability through the bond order

\frac{1}{2} (N_{b} - N_{a})

. For

H X_{2} X^{2 -}

the four electrons fill both the bonding and the antibonding

σ_{1 s}

orbitals, giving

\frac{1}{2} (2 - 2) = 0

, and a bond order of zero means no net bonding and so no viable molecule, whereas each of the other species keeps a positive bond order. The non-existent one is

H X_{2} X^{2 -}

JEE Main · 2018 · 08 Apr 2018 · Offline · Single correct option

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