1

### AIPMT 2008

Four diatomic species are listed below. Identify the correct order in which the bond order is increasing in them
A
$NO < {O_2}^ - < {C_2}^{2 - } < He_2^ +$
B
${O_2}^ - < NO < {C_2}^{2 - } < He_2^ +$
C
${C_2}^{2 - } < He_2^ + < {O_2}^ - < NO$
D
$He_2^ + < {O_2}^ - < NO < {C_2}^{2 - }$

## Explanation

Molecular orbital configuration of NO (15 electrons) is

${\sigma _{1{s^2}}}\,\sigma _{1{s^2}}^ * \,{\sigma _{2{s^2}}}\,\sigma _{2{s^2}}^ * \,{\sigma _{2p_z^2}}\,{\pi _{2p_x^2}}\, = \,{\pi _{2p_y^2}}\,\pi _{2p_x^1}^ * \, = \,\pi _{2p_y^o}^ *$

$\therefore\,\,\,\,$ Nb = 10

Na = 5

$\therefore\,\,\,\,$ BO = ${1 \over 2}\left[ {10 - 5} \right]$ = 2.5

Molecular orbital configuration of $O_2^ -$ (17 electrons) is

${\sigma _{1{s^2}}}\,\sigma _{1{s^2}}^ * \,{\sigma _{2{s^2}}}\,\sigma _{2{s^2}}^ * \,{\sigma _{2p_z^2}}\,{\pi _{2p_x^2}}\, = \,{\pi _{2p_y^2}}\,\pi _{2p_x^2}^ * \, = \,\pi _{2p_y^1}^ *$

$\therefore\,\,\,\,$ Nb = 10

Na = 7

$\therefore\,\,\,\,$ BO = ${1 \over 2}\left[ {10 - 7} \right]$ = 1.5

Molecular orbital configuration of $C_2^ {2-}$ (14 electrons) is

${\sigma _{1{s^2}}}\,\sigma _{1{s^2}}^ * \,{\sigma _{2{s^2}}}\,\sigma _{2{s^2}}^ * \,{\pi _{2p_x^2}}\, = \,{\pi _{2p_y^2}}\,{\sigma _{2p_z^2}}$

$\therefore\,\,\,\,$ Nb = 10

Na = 4

$\therefore\,\,\,\,$ BO = ${1 \over 2}\left[ {10 - 4} \right]$ = 3

$\,\,\,$ Configuration of $He_2^ +$ (3 electrons) is = ${\sigma _{1{s^2}}}$ $\sigma _{1{s^1}}^ *$

$\therefore\,\,\,$ Bond order = ${1 \over 2}$ (2 $-$1) = 0.5

$\therefore$ Correct order is :

$He_2^ + < {O_2}^ - < NO < {C_2}^{2 - }$
2

### AIPMT 2007

The correct order of C $-$ O bond length among CO, CO${_3^{2 - }}$, CO2 is
A
CO < CO${_3^{2 - }}$ < CO2
B
CO${_3^{2 - }}$ < CO2 < CO
C
CO < CO2 < CO${_3^{2 - }}$
D
CO2 < CO${_3^{2 - }}$ < CO

## Explanation

All these structures exhibits resonance and can be represented by the following resonating structures.

More single bond character in resonance hybrid, more is the bond length. Hence the increasing bond length is

CO $<$ CO2 $<$ CO${_3^{2 - }}$
3

### AIPMT 2007

In which of the following pairs, the two species are isostructural?
A
SO${_3^{2 - }}$ and NO${_3^{ - }}$
B
BF3 and NF3
C
BrO${_3^{ - }}$ and XeO3
D
SF4 and XeF4

## Explanation

Hybridisation of Br in BrO3 :

= ${1 \over 2}\left( {7 + 0 - 0 + 1} \right)$ = 4

Four hybrid orbital means sp3 hybridisation.

Hybridisation of Xe in XeO3 :

= ${1 \over 2}\left( {8 + 0 - 0 + 0} \right)$ = 4

Four hybrid orbital means sp3 hybridisation.

Thus, both BrO3 and XeO3 are sp3 hybridised with three bond pairs of electrons and one lone pair of electrons and results in trigonal pyramidal shape.

4

### AIPMT 2006

Which of the following species has a linear shape?
A
O3
B
NO2$-$
C
SO2
D
NO2+

## Explanation

Due to sp hybridisation of N+, NO2+ ion has linear shape.