Public Service Announcements

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Public Service Announcements

• Todaybegin discussion on bonding and shapehow
  theyre related
• Why is methane actually tetrahedral, not planar?
• Why is water bent, not linear
• Orbitalshow they morph from atomic to
  molecular
• Polarityhow to extend bond polarity to an entire
  molecule

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Polar Molecules and Dipole Moments

• A polar bond (Chapter 9) 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 (m) of a molecule is the
  product of the magnitude of the charge (d) and
  the distance (d) that separates the centers of
  positive and negative charge.
• m dd
• A unit of dipole moment is the debye (D).
• One debye (D) is equal to 3.34 x 1030 C m.

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• Example 10.4
• Explain whether you expect the following
  molecules to be polar or nonpolar.
• (a) CHCl3 (b) CCl4
• Example 10.5 A Conceptual Example
• Of the two compounds NOF and NO2F, one has m
  1.81 D and the other has m 0.47 D. Which
  dipole moment do you predict for each compound?
  Explain.

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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.

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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 (m 0 D).

No net dipole

• The water molecule has polar bonds also, but is
  an angular molecule.
• The bond dipoles do not cancel (m 1.84 D), so
  water is a polar molecule.

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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 electrons can also make a
contribution to dipole moments.
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How do bonds actually form?

• So far, weve only covered that bonds are formed
  when atoms share (or transfer) an electron(s).
• The space electrons occupyorbitals.
• Does our view of atomic orbitals mesh with VSEPR?
• Lets have a lookfor H2 bondingno problem

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Orbital shapesspherical, ps?

• S orbitalsno problem, p-orbitals? Recall,
  dumbbell shapeoriented 90 along x, y, z axes
• What about something simple like F2?
• HCl?? Again, no problem--even though its s and
  p orbital

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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 gt
• Ninety degreeshow
• Is that possible?

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Hybridizationthats how!

• Energy gap between s and p orbitals is low
• An electron absorbs energy and is promoted (2p)

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Promotionfollowed by hybridztn

• Remember that promotion is energy intensive,
  takes energyBUTnot as much energy saved when
  you can form additional bonds
• Making bonds RELEASES energy, breaking takes E

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Consistent with what weve learned?

• Yesremember that orbitals are mathematical
  equationsprobability of finding an electron
• When you combine several orbitals, the eq changes

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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 were dealing with an sp3
  orbital (one s and three p orbitals).

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Not all hybrids are bonding orbitals

• In each casemethane, ammonia, and water are all
  sp3 hybridized, but lone pairs of electrons
  occupy some of these orbitals

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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 isnt

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sp Hybridization in Be
with two unused p orbitals.
Two AOs combine to form
two hybrid AOs
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Lets relate EGG to hybrid geo!

• Note that the EGG and the hybrid orbital geometry
  are the same (which is why its often necessary
  to assign EGG in the first place).