A diatomic molecule, among the following species, that only has $\pi$ bonds according to the Molecular Orbital

Question

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

Accepted Answer

1. **Analyze Electron Configuration (MOT):** For diatomic molecules of elements with atomic number $Z \le 7$ (like Carbon, $Z=6$), the energy of $\pi_{2p}$ orbitals is lower than that of the $\sigma_{2p}$ orbital.
2. **Analyze $C_2$:**
   - Total electrons = 12.
   - Configuration: $\sigma_{1s}^2 \sigma_{1s}^{*2} \sigma_{2s}^2 \sigma_{2s}^{*2} (\pi_{2p_x}^2 = \pi_{2p_y}^2)$.
   - The last 4 electrons enter the degenerate $\pi$ bonding molecular orbitals.
   - **Bond Order** = $0.5 (N_b - N_a) = 0.5 (8 - 4) = 2$.
   - Since the bonding electrons involved in the valence shell (the outer 4) are strictly in $\pi$ orbitals, the double bond in $C_2$ consists of **two $\pi$ bonds** and no $\sigma$ bond.
3. **Compare others:**
   - $N_2$ (14e): One $\sigma$ and two $\pi$ bonds.
   - $O_2$ (16e): One $\sigma$ and one $\pi$ bond.
   - $Be_2$ (8e): Bond order is 0; does not exist.

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