Chapter 9 Molecular Structures

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Chapter 9Molecular Structures

• General Chemistry I
• T.Ara

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Drawing Lewis Structures

• Before we get started, work together to draw a
  valid Lewis structure for each of the formulas on
  p. 89 of your lab manual include lone pairs.
• The first three structures will have central
  atoms with less than an octet. Do not use
  multiple bonds.
• For each structure, fill in the table with the
  number of bonding electron pairs and the number
  of lone pairs around the central atom.

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A. Molecular Shapes

• The 3-dimensional shape of a molecule is part of
  what determines a molecules physical properties
• Boiling Point
• Melting Point
• Molecular shape also affects the biological
  activity of a molecule.
• Molecular formulas Lewis structures do not give
  information about molecular shape.

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1. Representing 3-D Molecules

• Use dashes wedges to draw 3-D molecules on flat
  surfaces.
• eg. Methane

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B. Valence Shell Electron Pair Repulsion (VSEPR)

• The VSEPR model predicts the shapes of molecules
  based on the fact that electrons repel each
  other.
• Lone pair bonding electrons are arranged such
  that they are as far apart as possible.
• The number of electron pairs around an atom
  dictates the most stable geometric arrangement of
  those electrons.

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1. Molecular Electron-Pair Geometry

• We will focus on two types of geometries
• Molecular Geometry the 3-dimensional arrangement
  of the atoms around a central atom
• Electron-Pair Geometry the geometry around a
  central atom, including the spatial positions of
  atoms and lone pair electrons
• For atoms with no lone pairs, the molecular
  electron-pair geometries are the same.

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2. Atoms with no Lone Pairs

• When an atom has no lone pairs, the molecular
  geometry can be predicted based on the number of
  atoms bonded to the central atom.
• Linear (2)
• Trigonal Planar (3)
• Tetrahedral (4)
• Trigonal Bipyramidal (5)
• Octahedral (6)

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a) Linear

• Atoms that are bonded to two other atoms have a
  linear geometry with a 180 bond angle.
• Bond Angle the angle
• between the bonds of two
• atoms that are bonded to
• the same third atom

Note The two oxygen atoms in CO2 are pointed in
opposite directions (as far apart as possible).
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b) Trigonal (Triangular) Planar

• Atoms that are bonded to three other atoms have a
  trigonal planar geometry with 120 bond angles.
• All four atoms are in the same plane.
• eg. AlCl3 is
• trigonal
• planar.

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c) Tetrahedral

• Atoms that are bonded to four other atoms have a
  tetrahedral geometry with 109.5 bond angles.
• eg. Silane has a
• tetrahedral geometry.

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Drawing 3-D Structures

• Draw the 3-D tetrahedral structure of methane
  using dashes and wedges.

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d) Trigonal (Triangular) Bipyramidal

• Atoms that are bonded to five other atoms have a
  trigonal bipyramidal geometry.
• Only atoms in the third row or lower can have
  this geometry because it requires an expanded
  valence.
• There are two distinct types of positions around
  the central atom
• Axial 90 from the equatorial atoms
• Equatorial 120 from each other
• eg. PF5 is trigonal bipyramidal.

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Drawing 3-D Structures

• Draw the 3-D trigonal bipyramidal structure of
  PCl5 using dashes and wedges.

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e) Octahedral

• Atoms that are bonded to six other atoms have an
  octahedral geometry.
• Only atoms in the third row or lower can have
  this geometry because it requires an expanded
  valence.
• All bond angles are 90.
• eg. SBr6 is octahedral.

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Drawing 3-D Structures

• Draw the 3-D octahedral structure of SF6 using
  dashes and wedges.

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Geometries Atoms with no Lone Pairs

• You should be able to predict and draw the
  geometry (with bond angles) of a central atom
  with no lone pairs.

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Predicting Geometries (no lone pairs)
Dichloromethane, a common organic solvent, has
the molecular formula CH2Cl2. Draw a Lewis
structure for dichloromethane, and use it to
predict and draw the 3-D molecular shape.
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Predicting Geometries (No Lone Pairs)
Borane has the molecular formula BH3. Draw a
Lewis structure for borane, and use it to predict
and draw the 3-D molecular shape. (Hint B has
less than an octet.)
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Predicting Geometries (No Lone Pairs)
Phosphorus pentafluoride has the molecular
formula PF5. Draw a Lewis structure for
phosphorus pentafluoride, and use it to predict
and draw the 3-D molecular shape.
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Central Atoms With Lone Pairs

• When the central atom has at least one lone pair
  of electrons, the molecular and the electron-pair
  geometries are different.
• How can we predict the geometries in these cases?

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3. Atoms Surrounded by 3 Electron Pairs

• REMEMBER If all three electron pairs are bonding
  electrons, the molecular and electron-pair
  geometries are the same trigonal planar.
• If one electron pair is nonbonding (1 lone pair)
• The electron-pair geometry is
• trigonal planar.
• The molecular geometry is
• angular (bent) - only pay
• attention to the relative
• orientation of the atoms, not
• the lone pairs.

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4. Atoms Surrounded by 4 Electron Pairs

• REMEMBER If all four electron pairs are bonding
  electrons, the molecular and electron-pair
  geometries are the same tetrahedral.
• If one electron pair is nonbonding (1 lone pair)
• The electron-pair geometry is tetrahedral.
• The molecular geometry is trigonal pyramidal.

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4. Atoms Surrounded by 4 Electron Pairs

• If two electron pairs are nonbonding (two lone
  pairs)
• The electron-pair geometry
• is tetrahedral.
• The molecular geometry is
• angular (bent).

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4. Atoms Surrounded by 4 Electron Pairs

• Lone pair electrons are bulkier than bonding
  electrons, so they take up more space.
• Notice the bond angle compression as lone pairs
  are added to the central atom

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Atoms Surrounded by 3 or 4 Electron Pairs

• Remember When the central atom has bonding and
  nonbonding electrons, it is important to
  distinguish between the molecular and
  electron-pair geometries.

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Drawing 3-D Structures

• Draw the 3-D structures of H2O and NH3 using
  dashes and wedges.

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Determining Geometry
Draw a Lewis structure for OCl2, and use it to
predict the molecular electron-pair geometries.
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5. Atoms Surrounded by 5 Electron Pairs

• REMEMBER If all five electron pairs are bonding
  electrons, the molecular and electron-pair
  geometries are the same trigonal bipyramidal.
• If one electron pair is nonbonding (one lone
  pair)
• The electron-pair
• geometry is trigonal
• bipyramidal.
• The molecular geometry
• is seesaw.

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5. Atoms Surrounded by 5 Electron Pairs

• Why does the lone pair prefer to be equatorial?
• Each axial position has a 90 bond angle with
  three other atoms/lone pairs.
• Each equatorial position has a 90 bond angle
  with only two other atoms/lone pairs.
• Bulky lone pairs prefer
• the less hindered equatorial
• positions.

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5. Atoms Surrounded by 5 Electron Pairs

• If two electron pairs are nonbonding (two lone
  pairs)
• The electron-pair
• geometry is trigonal
• bipyramidal.
• The molecular
• geometry is T-shaped.

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5. Atoms Surrounded by 5 Electron Pairs

• If three electron pairs are nonbonding (three
  lone pairs)
• The electron-pair geometry is trigonal
  bipyramidal.
• The molecular
• geometry is linear.
• Notice All of the equatorial
• positions are occupied by
• lone pairs.

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Drawing 3-D Structures

• Draw the 3-D structures of SF4, BrF3, and XeF2
  using dashes and wedges.

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Determining Geometry
Draw a Lewis structure for ClF2-, and use it to
predict the molecular electron-pair geometries.
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6. Atoms Surrounded by 6 Electron Pairs

• REMEMBER If all six electron pairs are bonding
  electrons, the molecular and electron-pair
  geometries are the same octahedral.
• If one electron pair is nonbonding (one lone
  pair)
• The electron-pair geometry is octahedral.
• The molecular geometry is square pyramidal.
• All positions are equivalent,
• so it doesnt matter where
• the lone pair goes.

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6. Atoms Surrounded by 6 Electron Pairs

• If two electron pairs are nonbonding (two lone
  pairs)
• The electron-pair geometry is octahedral.
• The molecular geometry is square planar.
• The two bulky lone
• pairs are pointed away
• from each other.

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Determining Geometry
Draw a Lewis structure for ICl5, and use it to
predict the molecular electron-pair geometries.
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6. Atoms Surrounded by 6 Electron Pairs
All of these geometries require an expanded
valence.
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7. Multiple Bonds Geometry

• For the purpose of determining molecular
  geometry, single and triple bonds are treated the
  same as single bonds.
• For example,
• formaldehyde
• borane are both
• trigonal planar with
• bond angles close to
• 120.

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Determining Geometry
Draw a Lewis structure for SO2, and use it to
predict the molecular electron-pair geometries.
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VSEPR Orbitals Do They Fit?

• As predicted by VSEPR, the most commonly observed
  bond angles are 180, 120, 109.5 90.
• How can we reconcile these observations with the
  shapes of the atomic orbitals?

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C. Hybridization

• In 1928, Linus Pauling proposed a bonding theory
  that uses atomic orbitals to explain the
  geometries predicted by VSEPR.
• In this theory, atomic orbitals (s, p, d) are
  combined to form hybrid atomic orbitals (sp, sp2,
  sp3, sp3d, sp3d2).
• The hybrid atomic orbitals on individual atoms
  then overlap with those on other atoms to form
  molecular bonding orbitals covalent bonds.

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1. Hybrid Orbitals

• Hybrid Orbitals orbitals resulting from the
  combination of atomic orbitals on the same atom
• Because of their shape, hybrid orbitals overlap
  more efficiently than unhybridized atomic
  orbitals to form stable covalent bonds with other
  atoms.
• A) Conservation of Orbitals The total number of
  hybrid orbitals formed is always equal to the
  number of atomic orbitals hybridized (combined).

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b) sp Hybrid Orbitals

• 1 s orbital 1 p orbital 2 sp orbitals
• The two sp orbitals are directed 180 from each
  other - can overlap with other atoms to form two
  bonds with a bond angle of 180.

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b) sp Hybrid Orbitals

• Before hybridization, Be has no unpaired
  electrons.
• After hybridization, Be can use both of its
  valence electrons to form covalent bonds.
• The energy it takes to hybridize the orbitals is
  made up for in the strong chemical bonds that are
  formed by the hybrid orbitals.

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b) sp Hybrid Orbitals

• Using its two sp hybrid orbitals, beryllium can
  form two single bonds.
• The bonding electrons reside in sigma bonding
  orbitals resulting from the overlap of the Be sp
  orbitals with the H 1s orbitals.

• Because of the orientation of the two sp
  orbitals, the geometry is linear (bond angle
  180?) SAME AS PREDICTED BY VSEPR!

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c) sp2 Hybrid Orbitals

• 1 s orbital 2 p orbitals 3 sp2 orbitals
• The three sp2 orbitals are directed 120 from
  each other - can overlap with other atoms to form
  three bonds with a bond angle of 120.

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c) sp2 Hybrid Orbitals

• Before hybridization, B has one unpaired
  electron.
• After hybridization, B can use all three of its
  valence electrons to form covalent bonds.

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c) sp2 Hybrid Orbitals

• Boron uses its three sp2 hybrid orbitals to form
  three single bonds (sigma bonding orbitals).
• The geometry is trigonal planar (bond angle
  120?).

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d) sp3 Hybrid Orbitals

• 1 s orbital 3 p orbitals 4 sp3 orbitals
• The four sp3 orbitals are directed 109.5 from
  each other - can overlap with other atoms to form
  four bonds with a bond angle of 109.5.

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d) sp3 Hybrid Orbitals

• Before hybridization, C has two unpaired
  electrons.
• After hybridization, C can use all four of its
  valence electrons to form covalent bonds.

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d) sp3 Hybrid Orbitals
- Carbon uses its four sp3 hybrid orbitals to
form four single bonds (sigma bonding
orbitals). - The geometry is tetrahedral (bond
angle 109.5?).
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e) sp3d Hybrid Orbitals

• Elements in the third period or lower can also
  hybridize their d orbitals.
• 1 s orbital 3 p orbitals 1 d orbital 5 sp3d
  orbitals
• The five sp3d orbitals are oriented in a trigonal
  bipyramidal geometry.

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f) sp3d2 Hybrid Orbitals

• 1 s orbital 3 p orbitals 2 d orbitals 6
  sp3d2 orbitals
• The six sp3d2 orbitals are oriented in an
  octahedral geometry.

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Hybrid Orbitals Expanded Valences

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2. Lone Pairs

• Nonbonding electrons (lone pairs) also occupy
  hybrid orbitals.

NH3 trigonal pyramidal
H2O angular (bent)
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3. Multiple Bonds

• Are multiple bonds (double, triple) also formed
  by the overlap of hybrid orbitals?
• NO - Multiple bonds are formed by the
  side-by-side overlap of unhybridized p orbitals.
• There are two types of covalent bonds
• Sigma formed from head-on overlap of hybrid
  orbitals
• Pi formed from side-by-side overlap of
  unhybridized p orbitals

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3. Multiple Bonds

• a) Sigma Bonds Sigma bonds are bonds formed by
  the head-on overlap of hybrid orbitals (all of
  the single bonds in a molecule)

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3. Multiple Bonds

• b) Pi Bonds Pi bonds are formed by the
  side-to-side overlap of unhybridized p orbitals
• Pi Bonding is used to form multiple bonds.

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3. Multiple Bonds

• The hybridization of the central atom determines
  how many p orbitals are left unhybridized (how
  many pi bonds can be formed by that atom).

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3. Multiple Bonds
-To form a multiple bond, an atom must have at
least one unhybridized p orbital available (sp,
sp2). -Atoms with sp3, sp3d sp3d2 hybridization
cannot form multiple bonds - no unhybridized p
orbitals.
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3. Multiple Bonds

• In a double bond, one electron pair forms a sigma
  bond one electron pair forms a pi bond.
• In a triple bond, one electron pair forms a sigma
  bond two electron pairs form two pi bonds.

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3. Multiple Bonds
A pi bond between two sp2-hybridized atoms in
formaldehyde.
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3. Multiple Bonds
Pi bonds between two sp-hybridized atoms in
acetylene.
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Sigma Pi Bonds

• To recap
• Single Bond one sigma bond
• Double Bond one sigma one pi bond
• Triple Bond one sigma two pi bonds

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Determining Hybridization from Lewis Structure

• Once you have drawn a valid Lewis structure for a
  molecule, there is a quick way to determine the
  hybridization (s, sp, sp2, etc.) of each atom in
  the molecule.
• Count the number of hybrid orbitals used by each
  atom.
• Remember Hybrid orbitals are used to form sigma
  bonds hold lone pairs.
• Hybrid Orbitals Sigma Bonds Lone Pairs

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Determining Hybridization from Lewis Structure
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Determining Hybridization from Lewis Structure
Determine the hybridization of the carbon atom
the bond angles in methane (CH4).
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Determining Hybridization from Lewis Structure
Determine the hybridization of the carbon atom
the bond angles in CO32-.
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Determining Hybridization from Lewis Structure
Determine the hybridization of the sulfur atom
the bond angles in SF6.
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Determining Hybridization from Lewis Structure
Determine the hybridization of each non-hydrogen
atom in the following molecule. Indicate the
bond angles.
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Determining Hybridization from Lewis Structure
Determine the hybridization of each non-hydrogen
atom in the following molecule. Indicate the
bond angles.
B
A
C
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Determining Hybridization from Lewis Structure
Determine the hybridization of each non-hydrogen
atom in the following molecule. Indicate the
bond angles.
E
D
C
B
A
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D. Molecular Polarity

• A molecule is polar when its polar bonds are
  arranged in such a way that the electron density
  is concentrated at one end of the molecule.
• The molecular dipole moment is the measure of the
  polarity of a molecule.
• The dipole moment
• measures the extent to
• which a molecule will line
• up with an external electric
• field (? attracted to and ?-
• attracted to ).

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D. Molecular Polarity

• Polar molecules have a permanent dipole moment
  greater than zero.
• Nonpolar molecules have a zero dipole moment.
• To determine whether a molecule is polar or
  nonpolar, you must consider
• Whether the molecule has any polar bonds.
• How the bonds are arranged (molecular geometry).

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D. Molecular Polarity

• When a molecule has only nonpolar bonds, it must
  be a nonpolar molecule.
• eg. Cl2 is a nonpolar
• molecule because it has
• one nonpolar bond - the
• bonding electrons are
• shared evenly by the two
• chlorine atoms.

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D. Molecular Polarity

• A molecule with polar bonds may or may not be
  polar.
• eg. HCl is polar because the HCl bond is polar
  the bonding electrons are shared unequally
  between the two atoms.

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1. Molecular Dipole Moment

• The molecular dipole moment is the sum of all of
  the individual bond dipoles geometry is
  important.
• In H2O CH3F, the bond dipoles reinforce each
  other, resulting in non-zero net dipole moments
  (polar).
• In CF4, the bond dipoles cancel each other out,
  resulting in no net dipole moment (nonpolar).

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1. Molecular Dipole Moment

• The shape of a molecule determines whether or not
  it has a molecular dipole moment.
• If the central atom of a molecule has only one
  type of substituent and the molecular geometry is
  one of the five ideal electron-pair geometries,
  then the molecule is nonpolar.

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1. Molecular Dipole Moment

• BF3 is trigonal planar, and all three bonds are
  to the same atom (F).
• This means that the dipoles cancel each other
  out.
• BF3 has no net dipole it is nonpolar.

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1. Molecular Dipole Moment

• NH3 is trigonal pyramidal not one of the ideal
  electron-pair geometries.
• The bond dipoles reinforce each other.
• NH3 has a net dipole it is polar.

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Determining Molecular Polarity
Is carbon dioxide (CO2) polar or nonpolar? Draw
the bond dipoles the molecular dipole (if any).
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Determining Molecular Polarity
Is PCl3 polar or nonpolar? Draw the bond dipoles
the molecular dipole (if any).
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Determining Molecular Polarity
Is SF4 polar or nonpolar? Draw the bond dipoles
the molecular dipole (if any).