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NEET CHEMISTRYChemical Bonding and Molecular StructureMedium

Question

A diatomic molecule, among the following species, that only has π\pi bonds according to the Molecular Orbital Theory (MOT) is:

A

Be₂

B

O₂

C

N₂

D

C₂

Step-by-Step Solution

  1. Analyze Electron Configuration (MOT): For diatomic molecules of elements with atomic number Z≤7Z \le 7 (like Carbon, Z=6Z=6), the energy of π2p\pi_{2p} orbitals is lower than that of the σ2p\sigma_{2p} orbital.
  2. Analyze C2C_2:
  • Total electrons = 12.
  • Configuration: σ1s2σ1s∗2σ2s2σ2s∗2(π2px2=π2py2)\sigma_{1s}^2 \sigma_{1s}^{*2} \sigma_{2s}^2 \sigma_{2s}^{*2} (\pi_{2p_x}^2 = \pi_{2p_y}^2).
  • The last 4 electrons enter the degenerate π\pi bonding molecular orbitals.
  • Bond Order = 0.5(Nb−Na)=0.5(8−4)=20.5 (N_b - N_a) = 0.5 (8 - 4) = 2.
  • Since the bonding electrons involved in the valence shell (the outer 4) are strictly in π\pi orbitals, the double bond in C2C_2 consists of two π\pi bonds and no σ\sigma bond.
  1. Compare others:
  • N2N_2 (14e): One σ\sigma and two π\pi bonds.
  • O2O_2 (16e): One σ\sigma and one π\pi bond.
  • Be2Be_2 (8e): Bond order is 0; does not exist.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Chemical Bonding and Molecular Structure. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

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