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1

AIPMT 2003

MCQ (Single Correct Answer)
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

MCQ (Single Correct Answer)
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

MCQ (Single Correct Answer)
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

MCQ (Single Correct Answer)
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.

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