Question

Which of the following represents the correct order of filling the molecular orbitals of $B_2,C_2$ and $N_2$ ?

• A. ${\pi }_{2p_x}={\pi }_{2p_y}<{\sigma }_{2p_z}<{\pi }_{2p_x}^{\ast }={\pi }_{2p_y}^{\ast }<{\sigma }_{2p_z}^{\ast }$
• B. ${\sigma }_{2p_z}<{\pi }_{2p_x}={\pi }_{2p_y}<{\pi }_{2p_x}^{\ast }={\pi }_{2p_y}^{\ast }<{\sigma }_{2p_z}^{\ast }$
• C. ${\sigma }_{2p_z}={\pi }_{2p_x}={\pi }_{2p_y}<{\pi }_{2p_x}^{\ast }={\pi }_{2p_y}^{\ast }<{\sigma }_{2p_z}^{\ast }$
• D. ${\pi }_{2p_x}<{\pi }_{2p_y}={\sigma }_{2p_z}>{\pi }_{2p_y}^{\ast }<{\pi }_{2p_z}^{\ast }={\sigma }_{2p_z}^{\ast }$

Answer 1

The correct option is A ${\pi }_{2p_x}={\pi }_{2p_y}<{\sigma }_{2p_z}<{\pi }_{2p_x}^{\ast }={\pi }_{2p_y}^{\ast }<{\sigma }_{2p_z}^{\ast }$

According to the modified modern concept, the energy of molecular orbitals increases in the order (number of electrons < 14)
$\sigma 1s<{\sigma }^{\ast }1s<\sigma 2s<{\sigma }^{\ast }2s<\pi 2p_x=\pi 2p_y<\sigma 2p_z<{\pi }^{\ast }2p_x={\pi }^{\ast }2p_y<{\sigma }^{\ast }2p_z$