NEET 2020 · ChemistryMO theoryPrevious Year Question

Identify a molecule which does not exist.

Answer: (C) $He X_{2}$ . \textbf{Answer:} (C) $He X_{2}$ \textbf{Solution:} By MOT, $He X_{2}$ has equal electrons in bonding and antibonding MOs, giving bond order $= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (2 - 2) = 0$ , so $He X_{2}$ does not exist.

A. $C X_{2}$
B. $O X_{2}$
C. $He X_{2}$ ✓
D. $Li X_{2}$

Correct Answer

Solution & Explanation

\textbf{Answer:} (C) $He X_{2}$ \textbf{Solution:} By MOT, $He X_{2}$ has equal electrons in bonding and antibonding MOs, giving bond order $= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (2 - 2) = 0$ , so $He X_{2}$ does not exist. $C X_{2}$ , $O X_{2}$ and $Li X_{2}$ have positive bond orders and do exist.

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