JEE Advanced · 2010 · 11 Apr 2010 · Paper 2 · Chemical Bonding and Molecular Structure

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

B_{2}

(A)1 and diamagnetic✓ Correct
(B)0 and diamagnetic
(C)1 and paramagnetic
(D)0 and paramagnetic

Answer: (A)

Solution

B₂ has 6 valence electrons. Normal MO configuration (following Hund's rule): σ1s² σ*1s² σ2s² σ*2s² π2p² (with 2 unpaired electrons in degenerate π orbitals), giving bond order 1 and paramagnetic. If Hund's rule is violated, the two π2p electrons pair up in one orbital: σ1s² σ*1s² σ2s² σ*2s² π2p² (both in same orbital). Bond order = (6-4)/2 = 1, and with all electrons paired, it becomes diamagnetic.

JEE Advanced · 2010 · 11 Apr 2010 · Paper 2 · Single correct option

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