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Which of the following would have a permanent dipole moment -
A
$BF_3$
B
$SF_4$
C
$SiF_4$
D
$XeF_4$
Detailed Solution
A molecule has a permanent dipole moment if its bond dipoles do not cancel, i.e. if the molecule is not symmetrical.
$BF_3$ is trigonal planar, $SiF_4$ is tetrahedral and $XeF_4$ is square planar; in all three the bond dipoles cancel by symmetry, so their dipole moment is zero.
$SF_4$: sulphur has four bond pairs and one lone pair ($sp^3d$). The lone pair occupies an equatorial position, so the molecule has a see-saw (distorted tetrahedral) shape.
In this unsymmetrical shape the bond dipoles do not cancel, and the lone pair also contributes, so $SF_4$ has a permanent dipole moment.
So the answer is $SF_4$.
$BF_3$ is trigonal planar, $SiF_4$ is tetrahedral and $XeF_4$ is square planar; in all three the bond dipoles cancel by symmetry, so their dipole moment is zero.
$SF_4$: sulphur has four bond pairs and one lone pair ($sp^3d$). The lone pair occupies an equatorial position, so the molecule has a see-saw (distorted tetrahedral) shape.
In this unsymmetrical shape the bond dipoles do not cancel, and the lone pair also contributes, so $SF_4$ has a permanent dipole moment.
So the answer is $SF_4$.