1

AIPMT 2004

Among the following, the pair in which the two species are not isostructural is
A
SiF4 and SF4
B
IO3$-$ and XeO3
C
BH4$-$ and NH4+
D
PF6$-$ and SF6

Explanation

SiF4 has symmetrical tetrahedral shape which is due to sp3 hybridisation of the central silicon atom in its excited state configuration.

SF4 has distorted tetrahedral or sea-saw geometry which arises due to sp3d hybridisation of the central sulphur atom and due to the presence of one lone pair of electrons in one of the equatorial hybrid orbital.
2

AIPMT 2004

H2O is dipolar, whereas BeF2 is not. It is because
A
the electronegativity of F is greater than that of O
B
H2O involves hydrogen bonding whereas BeF2 is a discrete molecule
C
H2O is linear and BeF2 is angular
D
H2O is angular and BeF2 is linear.

Explanation

In H2O the central atom oxygen has two lone pairs of electrons and two bond pairs of electrons. Thus, the shape of H2O is bent or V-shape. Thus, the two dipoles of H-O bonds have some resultant value as they are not exactly in the opposite direction to cancel out each other. While in BeF2, there are two bond pairs on central atom Be, hence it is linear in shape. Thus, the two dipoles of bonds are exactly in opposite direction to each other and cancel each other to give net zero dipole moment.
3

AIPMT 2004

In a regular octahedral molecule, MX6 the number of X $-$ M $-$ X bonds at 180o is
A
three
B
two
C
six
D
four

Explanation

The MX6 molecule with regular octahedral geometry is as follows :
Thus, there are three X—M—X bonds with bond angle 180o.
4

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.