1

### AIPMT 2012 Mains

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

Diatomic species Bond order
NO 2.5
$O_2^-$ 1.5
$C_2^{2-}$ 3.0
$He_2^+$ 0.5
The increasing order: $He_2^ + < {O_2}^ - < NO < {C_2}^{2 - }$
2

### AIPMT 2012 Mains

During change of O2 to O$_2^{ - }$ ion, the electron adds on which one of the following orbitals ?
A
${\pi ^ * }$ orbital
B
$\pi$ orbital
C
${\sigma ^ * }$ orbital
D
$\sigma$ orbital

## Explanation

Electronic configuration of O2

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

Thus the incoming electron will enter in $\pi ^*2p_x$ to form $O_2^+$
3

### AIPMT 2012 Prelims

Which of the following species contains three bond pairs and one lone pair arround the central atom ?
A
H2O
B
BF3
C
NH2$-$
D
PCl3

## Explanation

4

### AIPMT 2012 Prelims

Bond order of 1.5 is shown by
A
O2+
B
O2$-$
C
O22$-$
D
O2

## Explanation

Configuration of O2

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

Bond order = ${{No.\,of\,{e^ - }\,in\,bonding\,M.O - No.\,of\,{e^ - }\,in\,antibonding\,M.O} \over 2}$

Bond order of O2+ = ${{10 - 5} \over 2}$ = 2.5

Bond order of O2- = ${{10 - 7} \over 2}$ = 1.5

Bond order of O22- = ${{10 - 8} \over 2}$ = 1

Bond order of O2 = ${{10 - 6} \over 2}$ = 2