Question

The correct order of energies of molecular orbitals of N2 molecule, is

• A. σ1s<σ*1s<σ2s<σ*2s<σ2pz<(π2px =π2py)<(π*2px=π*2py)<σ*2pz
• B. σ1s<σ*1s<σ2s<σ*2s<σ2pz<σ*2pz<(π2px=π2py)<(π*2px=π*2py)
• C. σ1s<σ*1s<σ2s<σ*2s<(π2px=π2py)<(π*2px=π*2py)<σ2pz<σ*2pz
• D. σ1s<σ*1s<σ2s<σ*2s<(π2px=π2py)<σ2pz<(π*2px=π*2py)<σ*2pz

Answer 1

The Correct Option is D

The problem requires us to determine the correct order of molecular orbital energies for the nitrogen molecule (N₂). To solve this, we need to understand the molecular orbital theory, especially as it applies to diatomic molecules like N₂. The molecular orbital (MO) diagram helps us to predict the energy levels of the orbitals.

For diatomic molecules like N₂ (which have a lower atomic number, specifically lower than oxygen in the periodic table), the energy order of the molecular orbitals is slightly different from that in molecules like O₂ and F₂. This is due to the σ-π crossover phenomenon. For N₂ specifically, the order of orbitals is as follows:

1. σ1s
2. σ*1s
3. σ2s
4. σ*2s
5. π2p_x = π2p_y
6. σ2p_z
7. π*2p_x = π*2p_y
8. σ*2p_z

Let's break down why this is the correct order:

• The 1s orbitals (σ1s and σ*1s) are the core orbitals and are the lowest in energy.
• The 2s orbitals (σ2s and σ*2s) follow the 1s orbitals as they are the next available lower energy orbitals.
• For nitrogen (and lighter diatomic molecules), the π2p orbitals (π2p_x and π2p_y) are lower in energy than the σ2p_z orbital due to less effective overlap and lower energy.
• Next, we have the σ2p_z orbital, which has a higher energy level than the π2p orbitals because of greater overlap along the internuclear axis.
• Finally, the π*2p_x and π*2p_y antibonding orbitals precede the σ*2p_z antibonding orbital in terms of increasing energy because antibonding orbitals are generally higher in energy.

Hence, the correct order of energies of molecular orbitals of an N₂ molecule is:

σ1s < σ*1s < σ2s < σ*2s < (π2px = π2py) < σ2pz < (π*2px = π*2py) < σ*2pz