Assuming that Hund's Rule is violated, the bond order and magnetic nature of the diatomic molecule B_{2} is
a.1 and diamagnetic
b.0 and diamagnetic
c.1 and paramagnetic
d.0 and paramagnetic

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2 Answers
as there is no unpaired electron so it will be DIAMAGNETIC.
Bond Oreder = No.of bonding electrons  No.of antibonding electrons /2
i.e.= 64/2 =1
So the ans will be (a)1 and diamagnetic
Apply MOT here.
Step 1  Write down the actual distribution of electrons in the molecular orbitals for B_{2}
Step 2  You will find that the 10 electrons (5 each from each B atom) are distributed as:
Step 3  From here calculate the Bond Order (No. of electrons in Bonding  Antibonding Molecular Orbitals)/2 and magnetism (paramagnetic if there are unpaired electrons or else diamagnetic)
Step 4  Think of how this distribution of electrons would change if Hund's Rule is violated.
I have told you the approach. Now you let us know your answer.