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The correct order of energies of molecular orbitals of N₂ molecule, is:
For ≤ 14 electrons (up to N₂), sp-mixing elevates σ2p_z ABOVE π2p! The filling pattern is π before σ: π(2) – σ(1) – π*(2) – σ*(1). Option (2) in 10 seconds!
In homonuclear diatomic molecules, atomic orbitals of comparable energy and appropriate symmetry combine to form bonding (σ, π) and antibonding (σ*, π*) molecular orbitals. The internuclear axis is taken as the z-axis, so head-on overlap forms σ 2p_z, and lateral overlaps form degenerate π 2p_x and π 2p_y.
For elements up to nitrogen (Z ≤ 7, total electrons ≤ 14), the 2s and 2p atomic orbital energy levels lie close enough to undergo significant quantum mixing. This interaction raises the energy of the σ 2p_z orbital above the degenerate π 2p_x and π 2p_y orbitals:
Arranging all molecular orbitals in order of increasing energy:
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Places antibonding π* below bonding σ 2p_z, violating Aufbau principle.
Accurate energy sequence for N₂ with 2s-2p mixing.
This is the standard order for O₂ and F₂ (> 14 electrons) where NO s-p mixing occurs.
Unphysical order placing σ* below π.
In O₂ and F₂ (> 14 electrons), σ 2p_z is lower than π 2p (Option 3). But in N₂ (≤ 14 electrons), sp-mixing pushes σ 2p_z HIGHER than π 2p! Using the O₂ diagram for N₂ is the most common MOT blunder.
"For homonuclear diatomic molecules of second period elements with ≤ 14 electrons, the energy sequence is σ 1s < σ* 1s < σ 2s < σ* 2s < (π 2p_x = π 2p_y) < σ 2p_z < (π* 2p_x = π* 2p_y) < σ* 2p_z."
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