# Why is oxygen paramagnetic?

Paramagnetic molecules are molecules that have single electrons. When I draw the lewis structure of $\ce{O2}$, it appears to be a diamagnetic structure. What makes it paramagnetic?

• Perhaps you could show us the structure you are drawing? I don't think that Lewis structures are going to help you here because they hold no information about the spin of electrons.
– bon
May 3 '16 at 15:29
• You are right about the single electrons. But note the plural. What if you have two unpaired electrons and your molecule is a biradical? ;-) May 3 '16 at 15:46
• Is there even an answer on chem.SE explaining why O2 is paramagnetic? Everything I can find uses it as an example, but there's nothing explaining it... Edit: chemistry.stackexchange.com/a/39218/16683 The question isn't even about O2, sigh May 3 '16 at 16:00
• You might look at these links:-physlink.com/Education/AskExperts/ae493.cfm ......... chemistrynotmystery.blogspot.in/2014/09/… May 3 '16 at 16:05

To understand the paramagnetic nature of $\ce{O2}$, we must first understand how atomic orbitals mix together to form molecular orbitals. In the diatomic molecules of the elements in the second period, a phenomenon known as $\mathrm{s}$-$\mathrm{p}$ mixing results in an increase in the energy of the $\sigma_\mathrm{2p_z}$ molecular orbital, and a decrease in the energy of the $\sigma_\mathrm{2s}$ orbital. This is also observed in the $\sigma^{*}_\mathrm{2p_z}$ and $\sigma^{*}_\mathrm{2s}$ orbitals.
The degree of $\mathrm{s}$-$\mathrm{p}$ mixing is determined by the energy gap between the $\mathrm{s}$ and $\mathrm{p}$ orbitals. The higher the energy gap, the less the orbitals mix. Because the $\mathrm{s}$-$\mathrm{p}$ gap increases across a period, $\mathrm{s}$-$\mathrm{p}$ mixing in $\ce{O2}$ and $\ce{F2}$ is actually so low that the $\sigma^{*}_\mathrm{2p_z}$ orbital is lower in energy than the $\pi_\mathrm{2p_{x,y}}$ orbital, unlike the preceding elements.
By constructing the molecular orbital diagram for $\ce{O2}$ and filling each orbital according to Hund's rule, it becomes evident that $\ce{O2}$ is a diradical, with two unpaired electrons of the same spin. This is what gives oxygen its paramagnetism.