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1

AIPMT 2010 Mains

MCQ (Single Correct Answer)
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)
2

AIPMT 2010 Mains

MCQ (Single Correct Answer)
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 Prelims

MCQ (Single Correct Answer)
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.
4

AIPMT 2010 Prelims

MCQ (Single Correct Answer)
Which one of the following species does not exist under normal conditions?
A
Be2+
B
Be2
C
B2
D
Li2

Explanation

Be2 does not exists

Be2 has an electronic configuration of :

$$\sigma 1{s^2}{\sigma ^*}1{s^2}\sigma 2{s^2}{\sigma ^*}2{s^2}$$

$$ \therefore $$ Bond order = $${{4 - 4} \over 2} = 0$$

Thus, Be2 does not exists

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