Bonding in Some Homonuclear Diatomic Molecules

Applying LCAO and the Aufbau/Pauli/Hund filling rules, molecular orbital theory gives the electron configuration, bond order, and magnetic behaviour of the second-period homonuclear diatomic molecules, H₂ through O₂, as follows. (Note: for B₂, C₂ and N₂, the π2p molecular orbitals sit LOWER in energy than σ2p -- an ordering that FLIPS for O₂ and heavier diatomics, where σ2p sits lower than π2p; this subtlety is why B₂ turns out paramagnetic while O₂'s paramagnetism arises from a different, higher pair of orbitals, as detailed below.)

H₂ (Fig 10.30). Atomic configuration: H, 1s¹. Molecular-orbital configuration of H₂: $\sigma1s^2$. Bond order $= \dfrac{N_b - N_a}{2} = \dfrac{2 - 0}{2} = 1$. All electrons paired (no unpaired electrons) → diamagnetic.

Li₂ (Fig 10.31). Atomic configuration: Li, 1s² 2s¹. Molecular-orbital configuration of Li₂: $\sigma1s^2\,\sigma^{*}1s^2\,\sigma2s^2$. Bond order $= \dfrac{4 - 2}{2} = 1$. All electrons paired → diamagnetic.

B₂ (Fig 10.32). Atomic configuration: B, 1s² 2s² 2p¹. Molecular-orbital configuration of B₂: $\sigma1s^2\,\sigma^{*}1s^2\,\sigma2s^2\,\sigma^{*}2s^2\,\pi2p_y^1\,\pi2p_z^1$ (the two degenerate π2p orbitals are each singly filled, per Hund's rule, since for B₂ they sit LOWER in energy than σ2p). Bond order $= \dfrac{6 - 4}{2} = 1$. TWO unpaired electrons (one in π2p_y, one in π2p_z) → paramagnetic.

C₂ (Fig 10.33). Atomic configuration: C, 1s² 2s² 2p². Molecular-orbital configuration of C₂: $\sigma1s^2\,\sigma^{*}1s^2\,\sigma2s^2\,\sigma^{*}2s^2\,\pi2p_y^2\,\pi2p_z^2$. Bond order $= \dfrac{8 - 4}{2} = 2$. All electrons paired → diamagnetic.

N₂ (Fig 10.34). Atomic configuration: N, 1s² 2s² 2p³. Molecular-orbital configuration of N₂: $\sigma1s^2\,\sigma^{*}1s^2\,\sigma2s^2\,\sigma^{*}2s^2\,\pi2p_y^2\,\pi2p_z^2\,\sigma2p_x^2$. Bond order $= \dfrac{10 - 4}{2} = 3$. All electrons paired → diamagnetic (this triple bond and full bonding-orbital occupancy is why N₂ is so exceptionally stable/inert). …