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NEET CHEMISTRYEasy

A molecule that cannot exist is:

A

Li₂

B

C₂

C

O₂

D

He₂

Step-by-Step Solution

  1. Principle: According to Molecular Orbital Theory (MOT), a molecule is stable and can exist only if its Bond Order is positive (non-zero). If the bond order is zero, the attractive and repulsive forces cancel out, and no bond is formed.
  2. Bond Order Formula: Bond Order=12(Nb−Na)\text{Bond Order} = \frac{1}{2}(N_b - N_a), where NbN_b is the number of electrons in bonding orbitals and NaN_a is the number of electrons in anti-bonding orbitals.
  3. Analyze He₂:
  • Helium (Z=2Z=2) has electronic configuration 1s21s^2.
  • The molecule He2He_2 would have 2+2=42 + 2 = 4 electrons.
  • MOT Configuration: σ1s2σ1s∗2\sigma_{1s}^2 \sigma^*_{1s}{}^2.
  • Nb=2N_b = 2, Na=2N_a = 2.
  • Bond Order = 12(2−2)=0\frac{1}{2}(2 - 2) = 0.
  1. Conclusion: Since the bond order is zero, He2He_2 is unstable and cannot exist.
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