Download Chapter 9 Molecular Geometries and Bonding Theories

Molecular Geometries
and Bonding Theories
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Molecular Shapes
Lewis structure –
does not indicate
3D shape
Indicate
3D shape
• The shape of a
molecule plays an
important role in its
reactivity and
properties
• By noting the number
of bonding and
nonbonding electron
pairs we can easily
predict the shape of
the molecule.
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What Determines the Shape of a
Molecule?
• Bond angles! (the angles
between atoms)
• Electron pairs repel each
other
– bonding OR nonbonding
pairs
– So electron pairs are placed
as far as possible from each
other
– This lets us we can predict
the shape of the molecule
(ie, the angles between the
atoms)
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Electron Domains
• We can refer to the
electron pairs as electron
domains.
• Each of the following
counts as ONE electron
domain:
• The central atom in
this molecule, A, has
four electron
domains.
– A single bond
– A nonbonding pair (or lone
pair)
– A multiple bond (double or
triple bond)
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Valence Shell Electron Pair Repulsion
Theory (VSEPR)
“The best arrangement of a given number of
electron domains is the one that minimizes the
repulsions among them.”
In other words, the best structure is the one where
electron domains are most spread out.
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Electron-Domain Geometries
These are the
electron-domain
geometries for two
through six electron
domains around a
central atom.
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Electron-Domain Geometries
• Just count the number of electron domains in the
Lewis structure
• The electron-domain geometry will be that which
corresponds to the number of electron domains
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Molecular Geometries
• The electron-domain geometry is often not the shape of
the molecule, however.
• The molecular geometry is that defined by the positions
of only the atoms in the molecules, not the nonbonding
pairs.
• After finding electron-domain geometry, count the
bonding electron domains to determine molecular
geometry
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Linear Electron Domain
• In the linear domain, there is only one molecular
geometry: linear.
• NOTE: If there are only two atoms in the
molecule, the molecule will be linear no matter
what the electron domain is.
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Trigonal Planar Electron Domain
• There are two molecular geometries:
– Trigonal planar - if all the electron domains are
bonding
– Bent - if one of the domains is a nonbonding pair
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Tetrahedral Electron Domain
• For tetrahedral electron domain, there are three
molecular geometries:
– Tetrahedral - if all are bonding pairs
– Trigonal pyramidal - if one is a nonbonding pair
– Bent - if there are two nonbonding pairs.
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Trigonal Bipyramidal Electron Domain
• There are two distinct
positions in this
geometry:
– Axial
– Equatorial
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Trigonal Bipyramidal Electron Domain
Lower-energy conformations result from having
nonbonding electron pairs in equatorial, rather
than axial, positions in this geometry.
– In other words, nonbonding electrons will always
occupy equatorial positions
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Trigonal Bipyramidal Electron Domain
• There are four
distinct
molecular
geometries in
this domain:
– Trigonal
bipyramidal
– Seesaw
– T-shaped
– Linear
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Octahedral Electron Domain
• All positions are
equivalent in the
octahedral domain.
• There are three
molecular
geometries:
– Octahedral
– Square pyramidal
– Square planar
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Nonbonding Pairs and Bond Angles
• Nonbonding pairs are physically larger
than bonding pairs.
• Therefore, their repulsions are greater
• This tends to decrease bond angles in
a molecule.
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Multiple Bonds and Bond Angles
• Double and triple
bonds place greater
electron density on
one side of the central
atom than do single
bonds.
• Therefore, they also
affect bond angles.
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Larger Molecules
In larger molecules,
• We can assign a
geometry for each
“central” atom
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Polarity
Nonpolar molecule
• A bond dipole is due to unequal sharing
between 2 atoms in a covalent bond
– (a way to quantify bond polarity)
– shows the direction the electrons are being
pulled
• The overall dipole moment of a molecule is
a sum of its bond dipoles
• But just because a molecule possesses polar
bonds does not mean the molecule as a
whole will be polar.
• If the bond dipoles are equal in magnitude
but opposite in direction, they cancel each
other  nonpolar molecule
• If the overall dipole moment is not zero 
polar molecule
– In other words, if the central atom of the
molecule is symmetrically surrounded by
identical atoms, it will be nonpolar. Otherwise
it is polar.
Polar molecule
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Polarity
4.
1.
3.
2.
5.
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Examples
• Predict both the electron-domain geometry and the
molecular geometry for the following molecules. Finally,
indicate whether each is polar or nonpolar.
Electron-domain geometry
Molecular geometry
Polar/Nonpolar
a) FCl2+
b) AsF5
c) AsF3
d) ICl2e) TeF6
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Examples
• Predict both the electron-domain geometry and the
molecular geometry for the following molecules. Finally,
indicate whether each is polar or nonpolar.
Electron-domain geometry
Molecular geometry
Polar/Nonpolar
a) FCl2+
Trigonal planar
b) AsF5
Trigonal bipyramidal
c) AsF3
Tetrahedral
d) ICl2-
Trigonal bipyramidal
Linear
Nonpolar
e) TeF6
Octahedral
Octahedral
Nonpolar
Bent
Trigonal bipyramidal
Trigonal pyramidal
Polar
Nonpolar
Polar
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Overlap and Bonding
• We think of covalent bonds forming through the
sharing of electrons by adjacent atoms.
• This can only occur when orbitals on the two atoms
(called atomic orbitals) overlap (or share space).
H: 1s1
Cl: [Ne]3s23p5
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Overlap and Bonding
But simply
overlapping
atomic orbitals
doesn’t explain
the angles in
tetrahedral,
trigonal
bipyramidal, and
other
geometries.
The 3 p orbitals from the C atom
H
C H
H
H
≠
The
109.5°
angles
can’t be
explained
this way.
s orbital from each of 4 H atoms
The overlapped atomic orbitals are at 90° angles to each other here.
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Hybrid Orbitals
• So instead of simply overlapping, such atomic orbitals
hybridize (mix) to form new orbitals called hydrid
orbitals.
– Note: The total number of orbitals remains constant.
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Hybrid Orbitals
• Consider beryllium:
- In its ground electronic state,
it would not be able to form
bonds because it has no
unpaired electrons.
- But if it absorbs the small
amount of energy needed to
promote an electron from the
2s to the 2p orbital, it can
form two bonds because now
there are two unpaired
electrons.
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Hybrid Orbitals
Hybridize
• Mixing the s and p orbitals yields two degenerate
orbitals that are hybrids of the two orbitals.
– These sp hybrid orbitals have two lobes like a p orbital.
– One of the lobes is larger and more rounded (like an s
orbital).
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Hybrid Orbitals
• These two degenerate orbitals align themselves 180
from each other.
• This is consistent with the observed geometry of
beryllium compounds: linear.
– So a linear arrangement of electron domains implies sp
hybridization, and vice versa.
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Hybrid Orbitals
Using a similar model for boron leads to…
…three
degenerate
sp2
orbitals.
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Hybrid Orbitals
With carbon we get…
…four degenerate sp3 orbitals.
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Hybrid Orbitals
For geometries involving expanded octets on the
central atom, we must use d orbitals in our
hybrids.
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Hybrid Orbitals
This leads to five degenerate sp3d
orbitals…
…or six degenerate sp3d2 orbitals.
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Hybrid Orbitals
Once you know the
electron-domain
geometry, you know the
hybridization state of the
atom (and vice versa).
Steps to determining
hybridization of an atom:
1. Draw Lewis structure
2. Determine electrondomain geometry
3. Use this chart to
determine the
corresponding
hybridization of the
central atom.
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Examples
Complete the following chart:
Molecule or ion
Electron-domain
geometry
Hybridization of
central atom
Dipole moment?
Yes or no.
SiCl4
BrF4-
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Examples
Complete the following chart:
Molecule or ion
Electron-domain
geometry
Hybridization of
central atom
Dipole moment?
Yes or no.
SiCl4
Tetrahedral
sp3
No
BrF4-
Octahedral
sp3d2
No
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