JEE Advanced · 2014 · 25 May 2014 · Paper 2 · Chemical Bonding and Molecular Structure

Assuming

2 s - 2 p

mixing is NOT operative, the paramagnetic species among the following is

Answer: (C)

Solution

With

2 s

2 p

mixing switched off, the ordering places

σ_{2 p_{z}}

below the degenerate pair

π_{2 p_{x}}, π_{2 p_{y}}

. Counting valence electrons,

Be_{2}

is filled only through

σ_{2 s}^{*}

and

N_{2}

fills

σ_{2 p_{z}}

together with both

π_{2 p}

orbitals completely, so both are diamagnetic, and

B_{2}

with this ordering places its last two electrons paired in

σ_{2 p_{z}}

, again diamagnetic. For

C_{2}

, which has twelve electrons, the filling reaches

σ_{2 p_{z}}^{2}

and then puts the remaining two electrons singly into

π_{2 p_{x}}

and

π_{2 p_{y}}

by Hund's rule, leaving two unpaired electrons.

C_{2}

is therefore the paramagnetic species under this assumption.

JEE Advanced · 2014 · 25 May 2014 · Paper 2 · Single correct option