Polar Molecules and Dipole
Moments
• A polar bond (Chapter 10.1) has separate centers of
positive and negative charge.
• A molecule with separate centers of positive and negative
charge is a polar molecule.
• The dipole moment () of a molecule is the product of the
magnitude of the charge () and the distance (d) that
separates the centers of positive and negative charge.
 = d
• A unit of dipole moment is the debye (D).
• One debye (D) is equal to 3.34 x 10–30 C-m. (Coulomb-m)
Example
Explain whether you expect the following
molecules to be polar or nonpolar.
(a) CHCl3
(b) CCl4
Clearly polar: d- on Cl's
Polar bonds but
Symmetry makes
Overall molecule
nonpolar
A Conceptual Example
Of the two compounds NOF and NO2F, one
has  = 1.81 D and the other has  = 0.47 D.
Which dipole moment do you predict for each
compound? Explain.
Nitrosyl fluoride
NOF = ONF =
Nitryl fluoride
NO2F =
From Wayne
Breslyn
Drawing NO2F in Lewis Format
Note: would either
molecule show a
resonance
structure?
Bond Dipoles and Molecular
Dipoles
• A polar covalent bond has a bond dipole; a
separation of positive and negative charge
centers in an individual bond.
• Bond dipoles have both a magnitude and a
direction (they are vector quantities).
• Ordinarily, a polar molecule must have polar
bonds, BUT … polar bonds are not sufficient.
• A molecule may have polar bonds and be a
nonpolar molecule – IF the bond dipoles
cancel.
Bond Dipoles and Molecular
Dipoles
• CO2 has polar bonds, but
is a linear molecule; the
bond dipoles cancel and it
has no net dipole moment
( = 0 D).
• The water molecule has
polar bonds also, but is a
bent molecule.
• The bond dipoles do not
cancel ( = 1.84 D), so
water is a polar molecule.
No net
dipole
Net dipole
Molecular Shapes and Dipole
Moments
To predict molecular polarity:
1. Use electronegativity values to predict bond dipoles.
2. Use the VSEPR method to predict the molecular
shape.
3. From the molecular shape, determine whether bond
dipoles cancel to give a nonpolar molecule, or
combine to produce a resultant dipole moment for
the molecule.
Note: Lone-pair (unbonded) electrons can also
make a contribution to dipole moments.
How do bonds actually form?
• So far, we’ve only covered that bonds are
formed when atoms share (or transfer) an
electron(s).
– The space electrons occupy—orbitals.
– Does our view of atomic orbitals mesh with VSEPR?
• Let’s have a look…for H2 bonding…no problem
Orbital shapes…spherical, p’s?
• S orbitals…no problem, p-orbitals? Recall, dumbbell
shape—oriented 90 ° along x, y, z axes
– What about something simple like F2?
– HCl?? Again, no problem--even though it’s s and p orbital…
But those are simple molecules
• What about non-linear molecules?
– CH4?
– Ammonia
– Water.
• None are 90 °,
– P-orbitals ARE
– All 90 degrees
But these angles are >
Ninety degrees…how
Is that possible?
Hybridization…that’s how!
• Energy gap between s and p orbitals is low
– An electron absorbs energy and is promoted (2p)
Promotion—followed by hybridization
• Remember that promotion is energy intensive,
takes energy…BUT…not as much energy saved
when you can form additional bonds
– Making bonds RELEASES energy, breaking takes ΔE
Consistent with what we’ve learned?
• Yes…remember that orbitals are mathematical
‘equations’—probability of finding an electron
– When you combine several orbitals, the eq changes
This helps explain methane
• Also important to remember that the number of atomic
orbitals INTO a hybrid scheme MUST equal the number
of hybrid orbitals OUT of that scheme
– Previous example means we’re dealing with an sp3 orbital (one s
and three p orbitals).
Not all hybrids are bonding orbitals
• In each case…methane, ammonia, and water are all sp3
hybridized, but lone pairs of electrons occupy some of
these orbitals
What about other schemes
• Yep, can do those too. What about an sp2 scheme (one
s and two p orbitals)?
– 3 in, 3 out, all at 120° (just like Electron group geometry)
• One orbital isn’t
sp Hybridization in Be
… with two
unused p orbitals.
Two AOs combine to form …
… two hybrid AOs …
Electron-group geometry (EGG)
Electron-group geometry, abbreviated EGG,
is to electrons what VSEPR is to
molecules.
EGG tells us in what 3-D shape the electron
centers around an atom will group
themselves.
For example, the EGG arrangement around
carbon is often tetrahedral, if it is sp3
hybridized, as in methane, ethane, ethanol
Let’s relate EGG to hybrid geo!
• Note that the EGG and the hybrid orbital geometry are
the same (which is why it’s often necessary to assign
EGG in the first place).