IIT JEE Advanced 2010 Paper 2

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.