Question

Oxygen molecule is:

• A. diamagnetic
• B. paramagnetic
• C. ferromagnetic
• D. ferromagnetic

Answer 1

Answer: (B)

According to MOT (molecular orbital theory) oxygen (at. no. $8$) has the following orbital configuration
$O_2\left(16e^{-}\right)=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_z^2$
$\left(\pi 2p_x^2\pi 2p_y^2\right){\pi }^{\ast }2p_x^1{\pi }^{\ast }2p_y^1$
As oxygen has got two unpaired electrons, it is paramagnetic.
The sequence of energies of homonuclear diatomic molecule (except $O_2$ and $F_2$ ) is as follows
$\sigma 1s<{\sigma }^{\ast }1s<\sigma 2s<{\sigma }^{\ast }2s$
$<\left[\begin{array}{cc}\pi & 2p_x\\ \pi & 2p_y\end{array}\right]<\sigma 2p_z<\left[\begin{array}{cc}{\pi }^{\ast }& 2p_x\\ {\pi }^{\ast }& 2p_y\end{array}\right]<{\sigma }^{\ast }2p_z$
For oxygen and fluorine
$\sigma 1s<{\sigma }^{\ast }1s<\sigma 2s<{\sigma }^{\ast }2s<\sigma 2p_z$
$<\left[\begin{array}{cc}\pi & 2p_x\\ \pi & 2p_y\end{array}\right]<\left[\begin{array}{cc}\pi & 2p_x\\ \pi & 2p_y\end{array}\right]<{\sigma }^{\ast }2p_z$