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NEET CHEMISTRYMedium

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.
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