Using Molecular Orbital Theory explain why the `B_(2)` molecule is paramagnetic ?

Explanation

Electronic configuration of boron atom is `1s^(2)2s^(2)2p^(1)`. There are ten electrons in `B_(2)` molecule. In terms of molecular orbital theory the electronic configuration of `B_(2)` molecule is
`(sigma1s)^(2)(sigma^(star)1s)^(2)(sigma2s)^(2)(sigma^(star)2s)^(2)(pi2p^(1)=pi2p^(1))`
The presence of unpaired electrons in the two `pi` bonding molecular orbitals explains its paramagnetic nature.