Question

Give the correct increasing order of molecular orbital of $O_2$?

• A. $2\sigma ,2\pi ,2{\pi }^{\ast },2{\sigma }^{\ast }$
• B. $2\pi ,2\sigma ,2{\pi }^{\ast },2{\sigma }^{\ast }$
• C. $2\sigma ,2{\pi }^{\ast },2\pi ,2{\sigma }^{\ast }$
• D. $2\sigma ,2\pi ,2{\sigma }^{\ast },2{\pi }^{\ast }$

Answer 1

The correct option is A $2\sigma ,2\pi ,2{\pi }^{\ast },2{\sigma }^{\ast }$

All the elements in the second period before oxygen have the difference in energy between the $2s$ and $2p$ orbital small enough, so that $s-p$ mixing (combination) can occur lowering the energy of the $\sigma \left(2s\right)$ and $\sigma \ast \left(2s\right)$ and increasing the energy of the $\sigma \left(2p\right)$ and $\sigma \left(2p\right)$ molecular orbitals. By moving towards right in a period, the s orbital gets more stabilized than the $p$ orbital and the difference in their energies increases, making the $s-p$ mixing for oxygen much smaller.