Question

Oxygen molecule is paramagnetic because:
A) No. of bonding electrons $>$ no. of antibonding electrons.
B) No. of bonding electrons $<$ no. of antibonding electrons.
C) No. of bonding electrons $=$ no. of antibonding electrons.
D) Presence of unpaired electrons in molecular orbitals.

Answer 1

Presence of unpaired electrons in a molecule gives rise to paramagnetic state.
Molecular orbital theory is based on the concept of combination of atomic orbitals.

Complete answer:
Molecular orbital is formed due to the combination of atomic orbitals which gives rise to bonding state and antibonding state. The energy of the bonding state is lower than the antibonding state. So, the bonding states are filled first.
Oxygen molecules have a total of 16 electrons in it. According to molecular orbital theory, oxygen molecule has the following configuration:
$(\sigma 1{s^2})({\sigma ^ * }1{s^2})(\sigma 2{s^2})({\sigma ^ * }2{s^2})(\sigma 2p_z^2)(\pi 2p_x^2 = \pi 2p_y^2)({\pi ^ * }2p_x^1 = {\pi ^ * }2p_y^1)$
Here, we can see that the number of bonding electrons is 10 and the number of antibonding electrons is 6. So, from the above options, $\left( B \right)$ no. of No. of bonding electrons < no. of antibonding electrons and $\left( C \right)$ no. of bonding electrons = no. of antibonding electrons is wrong.
Though option $\left( A \right)$ no. of bonding electrons > no. of antibonding electrons is a correct statement, it has no relation to the formation of paramagnetism.
As we can see that there are two unpaired electrons in the antibonding state and presence of unpaired electrons give rise to a paramagnetic state, so the oxygen molecule is paramagnetic in nature.

Hence, the correct answer is (D) presence of unpaired electrons in molecular orbitals.

Note: The filling of the bonding state first is because of the Aufbau principle.
Each bonding or antibonding state has maximum two electrons to occupy due to the Pauli’s exclusion principle.
From molecular orbital theory, we can calculate the bond order, stability and symmetry of the molecules.