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NEET CHEMISTRYEasy

A molecule/ion among the following species that does not exist under normal conditions is:

A

Be₂⁺

B

Be₂

C

B₂

D

Li₂

Step-by-Step Solution

  1. Criterion for Existence: According to Molecular Orbital Theory (MOT), a molecule is viable only if its Bond Order is positive (greater than zero).
  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 bonding electrons and NaN_a is the number of antibonding electrons.
  3. Analyze Be₂:
  • Beryllium (Be, Z=4) has 4 electrons. The molecule Be2Be_2 has a total of 8 electrons.
  • Configuration: σ1s2σ1s∗2σ2s2σ2s∗2\sigma_{1s}^2 \sigma_{1s}^{*2} \sigma_{2s}^2 \sigma_{2s}^{*2}.
  • Nb=4N_b = 4 (22 in σ1s\sigma_{1s}, 22 in σ2s\sigma_{2s}).
  • Na=4N_a = 4 (22 in σ1s∗\sigma_{1s}^*, 22 in σ2s∗\sigma_{2s}^*).
  • Calculation: Bond Order=12(4−4)=0\text{Bond Order} = \frac{1}{2}(4 - 4) = 0.
  1. Conclusion: A bond order of zero implies no net bond forms between the atoms. Therefore, Be2Be_2 does not exist.
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