Molecular orbital theory

It's configuration is $\sigma_{1s^2} \sigma^{*}_{1s^2}\sigma_{2s^2} \sigma^{*}_{2s^2}\pi_{2p^1}\pi_{2p^1}$

$B_2$ is paramagnetic because it has two unpaired electrons..

DUE TO 2S-2P MIXING !

From the Molecular Orbital diagram, we can see that there is one one electron in both of the $\pi$ Bonds which results in Paramagnetic.

${ \sigma }_{ 1{ s }^{ 2 } }$ ${ \sigma * }_{ { 1s }^{ 2 } }$ ${ \sigma }_{ 2{ s }^{ 2 } }$ ${ \sigma * }_{ 2{ s }^{ 2 } }$ ${ \pi }_{ 2{ p }^{ 1 } }$ ${ \pi * }_{ 2{ p }^{ 1 } }$ MOT Diagram