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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

Chemical Bonding and Molecular Structure

Solution:

The molecular orbital electronic configuration of $\mathrm{B}_{2}$ molecule is
$\sigma 1 s^{2} \sigma^{*} 1 s^{2} \sigma 2 s^{2} \sigma^{*} 2 s^{2} \pi 2 p_{x}^{1} \pi 2 p_{y}^{1}$
If however, the Hund's rule is violated, then the configuration would be :
$\sigma 1 s^{2} \sigma^{*} 1 s^{2} \sigma 2 s^{2} \sigma^{*} 2 s^{2} \pi 2 p_{x}^{2}$
$\therefore \mathrm{B.O.}=\frac{6-4}{2}=1$
Since, all the electrons are paired up, the molecule is diamagnetic.