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Question:

In which of the following pairs of molecules/ions both the species are not likely to exist?

H₂,He₂

H₂,He₂⁺₂

H₂⁺₂,He₂

H₂⁺,He₂⁺₂

Solution:

For H₂, 1s²σ₁s¹ Bond Order = ½(2-0) = 1
For He₂, 1s²σ₁s²σ₁s*² Bond Order = ½(2-2) = 0
For He₂⁺₂, 1s²σ₁s² Bond Order = ½(2-0) = 1
For H₂⁺₂, 1s⁰ Bond Order = ½(0-0) = 0
For H₂, 1s²σ₁s¹ Bond Order = ½(2-0) = 1
For He₂⁺₂, 1s²σ₁s² Bond Order = ½(2-0) = 1
For H₂⁺, 1s¹ Bond Order = ½(1-0) = 0.5
The bond order of H₂⁺₂ and He₂ is zero, thus their existence is not possible. Hence, option C is correct.