1

### AIPMT 2003

Which one of the following statements is not correct for sigma- and pi- bonds formed between two carbon atoms?
A
sigma-bond is stronger than a pi-bond.
B
Bond energies of sigma- and pi-bonds are of the order of 264 kJ/mol and 347 kJ/mol, respectively
C
Free rotation of atoms about a sigma-bond is allowed but not in case of a pi-bond.
D
Sigma-bond determines the direction between carbon atoms but a pi-bond has no primary effect in this regard.

## Explanation

As sigma bond is stronger than the $\pi$(pi) bond, so it must be having higher bond energy than $\pi$(pi) bond.
2

### AIPMT 2002

Which of the following is isoelectronic?
A
CO2, NO2
B
NO2$-$, CO2
C
CN$-$, CO
D
SO2, CO2

## Explanation

In CO, the number of electrons

$\Rightarrow$ 6 + 8 = 14 [ Z of C = 6 and O = 8]

Electronic configuration of molecular orbital of CO :

${\left( {\sigma 1s} \right)^2}{\left( {{\sigma ^*}1s} \right)^2}{\left( {\sigma 2s} \right)^2}{\left( {{\sigma ^*}2s} \right)^2}{\left( {\pi 2{p_x}} \right)^2}{\left( {\pi 2{p_y}} \right)^2}{\left( {\pi 2{p_z}} \right)^2}$

CN- also get (6 + 7 + 1) 14 electrons and the configuration is similar to that CO.

So, CN and CO is isoelectronic.
3

### AIPMT 2002

In NO3$-$ ion number of bond pair and lone pair of electrons on nitrogen atom are
A
2, 2
B
3, 1
C
1, 3
D
4, 0

## Explanation

${O^ - }\hbox{---}\mathop N\limits^{\mathop \uparrow \limits^O } = O$

In NO3- ion, nitrogen has 4 bond pair of electrons and no lone pair of electrons.
4

### AIPMT 2002

Which of the following has p$\pi$ $-$ d$\pi$ bonding?
A
NO3$-$
B
SO32$-$
C
BO33$-$
D
CO32$-$

## Explanation

Electronic structure of S atom in excited state

In 'S' unhybride d- orbital is present, which will involved in bond formation with oxygen atom.

In oxygen two unpaired p- orbital is present in these one is involved in $\sigma$ bond formation while other is used in $\pi$ bond formation.

So, in SO32$-$, p$\pi$ $-$ d$\pi$ bonding present.