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If Hund's rule were not obeyed, what would be the bond order and magnetic behaviour of the B2 molecule?

1. 1 and diamagnetic
2. 0 and diamagnetic
3. 1 and paramagnetic
4. 0 and paramagnetic

Correct answer: 1 and diamagnetic

Solution

Normally B2 places its two pi(2p) electrons singly (paramagnetic, bond order 1). If Hund's rule is violated, both electrons pair in one pi orbital, leaving no unpaired electrons (diamagnetic) while the bond order stays 1.

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