Dipole Moments
How to tell if a molecule has a dipole moment
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© James B. Condon, 2007, all rights reserved
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Dipole Moments
A molecule has a dipole, when there is a charge
separation. The strength of the dipole depends
upon the difference in the electronegativity of the
atoms in the molecule.
A dipole is a charge separation in the molecule.
Dipole Moments
A molecule has a dipole, when there is a charge
separation. The strength of the dipole depends
upon the difference in the electronegativity of the
atoms in the molecule.
A dipole is a charge separation in the molecule.
A partial negative charge is sometimes given the
symbol δ– and a partial positive charge δ+.
Another way this is shown is with an arrow with a
cross on one end. The arrow indicate the direction
in which the electron concentrate.
Dipole Moments
The strength of the dipole depends upon the
difference in the electronegativity of the atoms in
the molecule.
For example: HCl has a dipole, since the electronegativity of H is 2.2 that of Cl is 3.16.
δ+ H-------Cl
δ–
Dipole Moments
The strength of the dipole depends upon the
difference in the electronegativity of the atoms in
the molecule.
For example: HCl has a dipole, since the electronegativity of H is 2.2 that of Cl is 3.16. Since Cl is
the more electronegative, electrons concentrate near
the Cl atom.
δ+ H-------Cl
δ–
Dipole Moments
Another example, CO2:
O is more electronegative, therefore there is a dipole
between the C atom and both O atoms.
O--------C--------O
Dipole Moments
Another example, CO2:
O is more electronegative, therefore there is a dipole
between the C atom and both O atoms.
Therefore, the C-O bonds have dipoles.
O--------C--------O
Dipole Moments
Another example, CO2:
The C-O bonds have dipoles, but does the overall
molecule have a dipole? (Is it polar?)
O--------C--------O
Dipole Moments
Another example, CO2:
Is CO2 polar?
CO2 is a linear molecule ( :O=C=O: with sp hybrid).
O--------C--------O
Dipole Moments
Another example, CO2:
Is CO2 polar? No, since the dipoles are in opposite
directions, the net effect is that they cancel. Thus,
the molecule does not have a dipole.
O--------C--------O
Dipole Moments
A third example, COS:
The Lewis dot structure is the same, :S=C=O:, so
the hybrid is sp and the molecule is linear.
S--------C--------O
Dipole Moments
A third example, COS:
The molecule is linear, however, the C-O bond is
much more dipolar than the C-S bond.
S--------C--------O
Dipole Moments
A third example, COS:
The molecule is linear, however, the C-O bond is
much more dipolar than the C-S bond.
The net effect is a polar molecule
S--------C--------O
Addition of the dipoles
yields a net dipole. i.e. =
Dipole Moments
A 4th example, H2O:
The molecule is bent 109E with 2 lone pair due to the
sp3 hybrid formation.
O
H
H
Dipole Moments
A 4th example, H2O:
The molecule is bent 109E with 2 lone pair.
O is more electronegative than H and therefore the
bond has a dipole.
But is there a net dipole?
O
H
H
Dipole Moments
A 4th example, H2O:
The molecule is bent 109E with 2 lone pair.
Is there a net dipole? Yes, the dipoles do not cancel
completely due to the bent shape. There is a net
dipole toward the O end of the molecule.
=
O
H
H
Dipole Moments
A 4th example, CH4:
The hybridization about C is sp3 and the overall
molecular geometry is tetrahedral.
H
H
C
H
H
Dipole Moments
A 4th example, CH4:
The hybridization about C is sp3 and the overall
molecular geometry is tetrahedral. The C is slightly
more electronegative than H, therefore the bonds
are polar.
H
H
C
H
H
Dipole Moments
A 4th example, CH4:
The molecular geometry is tetrahedral.
Even though the bonds are polar, the overall effect is
that there is no net dipole. This is difficult to see in a
2D picture, but more obvious with a 3D model.
H
H
C
H
H
Dipole Moments
A 3D model of CH4:
Dipole Moments
A 4th example, CH4:
The molecular geometry is tetrahedral.
As a general rule, if the 3D model is out of balance,
then the molecule is polar, but if it is in balance as
illustrated in the previous slide, then it is non-polar.
H
H
C
H
H
Dipole Moments
Examples 5 and 6: If 1 or 2 of the terminal atoms in
the tetrahedron are different from the others, the
molecule is polar.
Examples would be CH2Cl2 and CH3Cl.
H
H
H
C
C
H
H
Cl
Cl
Cl
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar?
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar?
These are both resonence structures with sp2
hybridization:
– : ø :O–S–O
–
:O–S–O
:
:O:
:O:
O
– : ø:O–S–O
:O–S–O
–
:ø :O–S–O:
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar? Both have sp2 hybridization:
SO2 is bent 120E and SO3 is trigonal.
– : ø :O–S–O
–
:O–S–O
: Bent 120E
:O:
:O:
O
– : ø:O–S–O
:O–S–O
–
:ø :O–S–O: Trigonal
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar? Both have sp2 hybridization:
SO2 is bent 120E and SO3 is trigonal. The stickfigure representations would be:
O
S
O
O
S
O
O
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar? Both have sp2 hybridization:
... and the dipole on the bonds would be:
O
S
O
O
S
O
O
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar? Both have sp2 hybridization:
For SO2 there is a net dipole since the dipoles do not
completely cancel.
O
=
S
O
O
S
O
O
Dipole Moments
Examples 7 and 8 sp2 hybridization - two contrasting
examples:
Are SO2 and SO3 polar? Both have sp2 hybridization:
For SO3 on the other hand the dipoles do completely
cancel (are balanced) so there is no net dipole.
O
=
S
O
O
S
O
= nothing
O
Dipole Moments
Higher geometries - sp3d hybrid:
Two examples of sp3d hybrid - PF5 and PF4– ion.
The stick figures are as shown:
F
F
F
P
F
PF5
F
F
F
F
P :
F
–
PF4
Dipole Moments
Higher geometries - sp3d hybrid:
Two examples of sp3d hybrid - PF5 and PF4– ion.
Notice that in both cases the F above and below
balance yielding no net dipole in the vertical direction.
F
F
F
P
F
PF5
F
F
F
F
P :
F
–
PF4
Dipole Moments
Higher geometries - sp3d hybrid:
Two examples of sp3d hybrid - PF5 and PF4– ion.
The Fs about the waist cancel for PF5 but not for PF4–
F
F
F
P
F
PF5
F
F
F
F
P :
F
–
PF4
Dipole Moments
Higher geometries - sp3d hybrid:
Two examples of sp3d hybrid - PF5 and PF4– ion.
Therefore, PF5 does not have a dipole but PF4– does.
F
F
F
P
F
PF5
F
F =0
F
F
P :
F
–
PF4
=
Dipole Moments
Higher geometries - sp3d hybrid:
Another example of sp3d hybrid - PF3Cl2
There are 3 possible arrangements for PF3Cl2. The
stick figures for these are shown below.
Cl
Cl
F
F
P
Cl
F
F
F
P
F
F
Cl
F
Cl
P
F
Cl
Dipole Moments
Higher geometries - sp3d hybrid:
Another example of sp3d hybrid - PF3Cl2
Notice that only the first one is balanced and
therefore has no dipole. The others are polar.
Cl
Cl
F
F
P
Cl
F
F
F
P
F
F
Cl
F
Cl
P
F
Cl
Dipole Moments
Higher geometries - sp3d2 hybrid:
Contrast SF6 with ClF5. Both of these have an sp3d2
hybrid about the central atom.
SF6 has an atom on each branch and is therefore
octahedral.
F
F
F
S
F
F
F
F
F
F
Cl
F
F
Dipole Moments
Higher geometries - sp3d2 hybrid:
Contrast SF6 with ClF5. Both of these have an sp3d2
hybrid about the central atom.
ClF5, however, has a lone pair of electrons and is a
trigonal pyramid.
F
F
F
S
F
F
F
F
F
F
Cl
F
F
Dipole Moments
High geometries - sp3d2 hybrid:
Contrast SF6 with ClF5. Both of these have an sp3d2
hybrid about the central atom.
SF6 is completely in balance, therefore it has no
dipole.
F
F
F
S
F
F
F
F
=0
F
F
Cl
F
F
Dipole Moments
High geometries - sp3d2 hybrid:
Contrast SF6 with ClF5. Both of these have an sp3d2
hybrid about the central atom.
ClF5, however, is out of balance, and therefore does
have a dipole.
F
F
F
S
F
F
F
F
=0
F
F
Cl
F
F =
Dipole Moments
Some generalization about dipole for molecules
with 1 central atom for some cases
Dipole Moments
Some generalization about dipole for molecules with
1 central atom for some cases
M If all the terminal positions are identical -there is no dipole
Dipole Moments
Some generalization about dipole for molecules with
1 central atom for some cases
M If all the terminal positions are identical -there is no dipole
M If only one terminal position in the hybrid differs from the
others - there is a dipole
Dipole Moments
Some generalization about dipole for molecules with
1 central atom for some cases
M If all the terminal positions are identical -there is no dipole
M If only one terminal position in the hybrid differs from the
others -there is a dipole
M for the sp3 hybrid, all terminal atoms must be identical for
no dipole, otherwise there is a dipole
Dipole Moments
Some generalization about dipole for molecules
with 1 central atom for some cases
M If all the terminal positions are identical -there is no dipole
M If only one terminal position in the hybrid differs from the
others -there is a dipole
M for the sp3 hybrid, all terminal atoms must be identical for
no dipole, otherwise there is a dipole
The above do not cover all cases. All other cases
must be determined by considering the specific
geometry.
The End of the slideshow
“Dipole Moments”
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