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Question:

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

0 and diamagnetic

1 and diamagnetic

0 and paramagnetic

1 and paramagnetic

Solution:

In Hund's rule of maximum multiplicity, if two or more orbitals of equal energy are available, electrons will occupy them singly before filling them in pairs.
B2(10)=σ1s²σ1s²σ2s²σ2s²π2px²
Number of bonding electrons=6
Number of Antibonding electrons=4
Bond order=(Number of bonding electrons - Number of antibonding electrons)/2
Bond order=6-4/2=1
Since there are no unpaired electrons, it is diamagnetic in nature.