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  4. Assuming that Hund's rule is violated, the bond order and magnetic nature of the diatomic molecule B2 is

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Q. Assuming that Hund's rule is violated, the bond order and magnetic nature of the diatomic molecule $B_2$ is

IIT JEEIIT JEE 2010Chemical Bonding and Molecular Structure

Solution:

For molecules lighter than $O _{2}$, the increasing order of energies of molecular orbitals is
image
where, $\pi 2 p_{y}$ and $\pi 2 p_{z}$ are degenerate molecular orbitals, first singly occupied and then pairing starts if Hund's rule is obeyed. If Hund's rule is violated in $B_{2}$, electronic arrangement would be
image
No unpaired electron-diamagnetic.
Bond order $=\frac{\text { bonding electrons }-\text { antibonding electrons }}{2} $
$=\frac{6-4}{2}=1$