1. He ₂ 2. Li ₂ 3. C ₂ 4. O ₂

Answer: 1. He ₂ A molecule exists only if its bond order (from molecular orbital theory) is greater than zero. He₂ : 4 electrons, σ1s²σ^*1s² , bond order =(2-2)/2=0 . It does not exist. Li₂ : bond order 1. C₂ : bond order 2. O₂ : bond order 2. So the answer is He₂ .

Topic: Molecular orbital theory, bond order, bond length, bond energy· Chemical Bonding and Molecular Structure

Identify a molecule which does not exist.

1. He\(_2\)

2. Li\(_2\)

3. C\(_2\)

4. O\(_2\)

Doubtly Asked 30 Oct 2026 NEET UG 2020 Practice test neet Save WhatsApp Report

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Answer: 1. He\(_2\)

A molecule exists only if its bond order (from molecular orbital theory) is greater than zero.

\(\mathrm{He_2}\): 4 electrons, \(\sigma1s^2\sigma^*1s^2\), bond order \(=(2-2)/2=0\). It does not exist.
\(\mathrm{Li_2}\): bond order 1. \(\mathrm{C_2}\): bond order 2. \(\mathrm{O_2}\): bond order 2.

So the answer is \(\mathrm{He_2}\).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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