Question

The number of unpaired electrons in $O_2$ molecule is:

• A. Zero
• B. 1
• C. 2
• D. 3

Answer 1

The correct option is C 2

Oxygen molecule $\left(O_2\right):$ Each oxygen atom has 8 electrons. Hence, $O_2$ molecule has a total of 16 electrons. The electronic configuration of $O_2$ molecule therefore is:
$O_2:\left(\sigma 1s{)}^2\right({\sigma }^{\ast }1s{)}^2\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\sigma 2p_z{)}^2\right(\pi 2p_x{)}^2=\left(\pi 2p_y{)}^2\right({\pi }^{\ast }2p_x{)}^1=({\pi }^{\ast }2p_y{)}^1$

As we can see from the above electronic configuration, there are two unpaired electrons in the antibonding molecular $\pi$ -orbital which leads to oxygen exhibiting paramagnetic properties.