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Question:

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

H2⁻

H⁻

He2⁺

He⁺

Solution:

Electronic configuration of H₂⁻ = (σ1s)²(σ*1s)²
Bond order of H₂⁻ = (Nb - Na)/2 = 2 - 2/2 = 0.
If a bond order of zero is obtained, that means that the molecule is too unstable and so it will not exist. Hence, H₂⁻ does not exist, due to zero bond order. Option B is correct.