For B2, the molecular orbital configuration is B2(10) : σ1s2σ*1s2σ2s2σ*2s2π2px1π2py1 Since the Hund’s rule is violated, two electrons are placed in π2px molecular orbital, so B2(10) : σ1s2σ*1s2σ2s2σ*2s2π2px2 Thus, bond order =
6−4
2
= 1. As there are no unpaired electrons, the nature is diamagnetic.