1

### AIPMT 2011 Prelims

Which of the following has the minimum bond length ?
A
O2+
B
O2$-$
C
O22$-$
D
O2

## Explanation

Electronic configuration
O2 : $\sigma 1{s^2}{\sigma ^*}1{s^2}\sigma 2{s^2}{\sigma ^*}2{s^2}\sigma 2p_z^2\pi 2p_y^2{\pi ^*}2p_y^1{\pi ^*}2p_z^1$

Bond order = ${1 \over 2}\left( {8 - 4} \right)$ = 2

O2+ : Bond order = ${1 \over 2}\left( {8 - 3} \right)$ = 2.5

O2- : Bond order = ${1 \over 2}\left( {8 - 5} \right)$ = 1.5

O22- : Bond order = ${1 \over 2}\left( {8 - 6} \right)$ = 1

As bond order increases, bond length decreases.
2

### AIPMT 2010 Mains

Some of the properties of the two species, $NO_3^ -$ and ${H_3}{O^ + }$ are described below. Which one of them is current?
A
Dissimilar in hybridization for the central atom with different structures.
B
Isostructural with same hybridization for the central atom.
C
Isostructural with different hybridization for the central atom
D
Similar in hybridization for the central atom with different structures.

## Explanation

No. of electron pairs = No. of atoms bonded to it + 1/2[Gp. no. of central atom - Valency of central atom $\pm$ No. of electrons]

For NO3- = 3 + 1/2[5 - 6 + 1] = 3 (sp2 hybridisation)

For H3O+ = 3 + 1/2[6 - 3 - 1] = 4 (sp3 hybridisation)
3

### AIPMT 2010 Mains

In which of the following molecules the central atom does not have sp3 hybridization?
A
CH4
B
SF4
C
BF4$-$
D
NH4+

## Explanation

For neutral molecules

No. of electron pairs = No. of atoms bonded to it + 1/2[Gp. no. of central atom - Valency of central atom]

$\therefore$For CH4, no. of e- pairs = 4 + 1/2[4 - 4] = 4 (sp3 hybridisation)

For SF4, no. of e- pairs = 4 + 1/2[6 - 4] = 5 (sp3d hybridisation)

and For ions,

No. of electron pairs = No. of atoms bonded to it + 1/2[Gp. no. of central atom - Valency of central atom $\pm$ No. of electrons]

$\therefore$For BF4-, no. of e- pairs = 4 + 1/2[3 - 4 + 1] = 4 (sp3 hybridisation)

For NH4+, no. of e- pairs = 4 + 1/2[5 - 4 - 1] = 4 (sp3 hybridisation)
4

### AIPMT 2010 Prelims

In which one of the following species the central atom has the type of hybridization which is not the same as that present in the other three ?
A
SF4
B
I3$-$
C
SbCl52$-$
D
PCl5

## Explanation

The hybridisation of the central atom can be calculated as

H = ${1 \over 2}\left[ \matrix{ \left( \matrix{ No.\,of\,electrons \hfill \cr in\,valence\,shell \hfill \cr of\,atom \hfill \cr} \right) + \left( \matrix{ No.\,of\,monovalent \hfill \cr atoms\,around \hfill \cr central\,atom \hfill \cr} \right) \hfill \cr - \left( \matrix{ Ch{\mathop{\rm arge}\nolimits} \,on \hfill \cr cation \hfill \cr} \right) + \left( \matrix{ Ch{\mathop{\rm arge}\nolimits} \,on \hfill \cr anion \hfill \cr} \right) \hfill \cr} \right]$

Applying that formula we find that all the given species except [SbCl5]2- have central atom with sp3d ( corresponding to H = 5) hybridisation. In [SbCl5]2- Sb is sp3d2 hybridized.

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