The paramagnetic behavior of B₂ is due to the presence of

Number of electrons in B₂ is 10 and its molecular orbital electronic configuration is:

$\left(\mathrm{\sigma }1{\mathrm{s}}^2\right)\left(\mathrm{\sigma }*1{\mathrm{s}}^2\right)\left(\mathrm{\sigma }2{\mathrm{s}}^2\right)\left(\mathrm{\sigma }*2{\mathrm{s}}^2\right)\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}^1=\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}^1\right)$

The paramagnetic behavior of B₂ is due to the presence of two unpaired electrons in $\mathrm{\pi }-$bonding molecular orbital of B₂.