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 molecular orbital (MO) energy ordering for diatomic molecules like N₂ can be understood by considering the interaction between atomic orbitals. In diatomic molecules, energy levels are determined by the molecular orbital theory which involves the combination of atomic orbitals to form molecular orbitals. For molecules with less than or equal to 14 electrons, like N₂, the order of energy levels is different from those with more than 14 electrons. 

Steps to determine the correct order:

1. Lower energy MOs: Start with the 1s orbitals. The molecular orbitals formed are σ1s and σ*1s, with σ1s being lower in energy than σ*1s.
2. 2s orbitals: The next higher energy levels are formed by the 2s orbitals, resulting in σ2s and σ*2s, with σ2s being at a lower energy than σ*2s.
3. 2p orbitals: When constructing MOs from p orbitals in second-period diatomic molecules:
   • (π2p_x = π2p_y) has lower energy than σ2p_z due to the node-related distribution in π orbitals offering slightly lower energy orientation.
   • σ2p_z has higher energy than (π2p_x = π2p_y), but less than anti-bonding π orbitals (π*2p_x = π*2p_y).
   • (π*2p_x = π*2p_y) are filled after σ2p_z and are higher in energy.
   • Finally, σ*2p_z is the highest energy orbital in these configurations.

Combining these observations, the correct order of molecular orbitals for N₂ is:

σ1s < σ*1s < σ2s < σ*2s < (π2p_x = π2p_y) < σ2p_z < (π*2p_x = π*2p_y) < σ*2p_z

Answer 2

• The molecular orbitals are arranged in increasing energy levels.
• The σ1s orbital is the lowest in energy as it is the bonding molecular orbital formed by the overlap of two 1s atomic orbitals.
• The σ*1s orbital is the next in energy, representing the antibonding orbital formed by the out-of-phase overlap of two 1s atomic orbitals.
• The σ2s orbital is lower in energy than the σ*2s orbital, as the σ2s is formed by the overlap of two 2s atomic orbitals.
• The (π2px = π2py) orbitals are degenerate and have the same energy level. These are the bonding and antibonding π molecular orbitals formed by the overlap of two 2px and 2py atomic orbitals.
• The σ2pz orbital is lower in energy than the (π2px = π2py) orbitals, as the σ2pz is formed by the overlap of two 2pz atomic orbitals.
• Finally, the (π2px = π2py) orbitals are the highest in energy among these molecular orbitals.

Therefore, The correct option is (D): σ1s<σ*1s<σ2s<σ*2s<(π2px=π2py)<σ2pz<(π*2px=π*2py)<σ*2pz