For molecules lighter than O2​, the increasing order of energies of molecular orbitals iswhere, π2py​ and π2pz​ are degenerate molecular orbitals, first singly occupied, and then pairing starts if Hund's rule is obeyed. If Hund's rule is violated in B2​, electronic arrangement would beNo unpaired electron-diamagnetic. Bond order ​=2 bonding electrons − antibonding electrons ​=26−4​=1​