Transcript Chapter 9 Molecular Geometries and Bonding Theories

Chapter 9
Molecular Geometries
and Bonding Theories
Molecular
Geometries
and Bonding
November 23
ONLINE PRACTICE QUESTIONS
UNIT 8 – BY NEXT SUNDAY!!!
WATCH PODCASTS 8-3 is this lesson
8-1 and 8-2 review of chapter 8
• Molecular Shapes
• VSEPR theory
• Electron Domain Geometry
• Molecular Geometry
Molecular
Geometries
• Hw for section 9.1 and 9.2 : 1,2,3,11 toand
Bonding
23 odd
What Determines the Shape of a
Molecule?
• The Lewis structure is drawn with the
atoms all in the same plane.
• The overall shape of the molecule will
be determined by its bond angles.
Molecular
Geometries
and Bonding
What Determines the Shape of a
Molecule?
• Electron pairs, whether
they be bonding or
nonbonding, repel each
other.
• By assuming the electron
pairs are placed as far as
possible from each other,
we can predict the shape
of the molecule.
Molecular
Geometries
and Bonding
There are
five
fundamental
geometries
for
molecular
shape
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
Electron Domains
• This molecule has
four electron
domains.
• We can refer to the
electron pairs as electron
domains.
• Each pair of electrons
count as an electron
domain, whether they are
in a lone pair, in a single,
double or triple bond.
Molecular
Geometries
and Bonding
Electron-Domain Geometries
• All one must do is
count the number of
electron domains in
the Lewis structure.
• The geometry will
be that which
corresponds to that
number of electron
domains.
Molecular
Geometries
and Bonding
In order to predict molecular shape, we assume the
valence electrons repel each other. Therefore, the
molecule adopts whichever 3D geometry minimizes this
repulsion.
• We call this process Valence
Shell Electron Pair
Repulsion (VSEPR) theory.
• There are simple shapes for AB2 to AB6 molecules.
Molecular
Geometries
and Bonding
• Electron domain geometry: When considering the
geometry about the central atom, we consider all
electrons (lone pairs and bonding pairs).
• When naming the molecular geometry, we focus
only on the positions of the atoms.
Molecular
Geometries
and Bonding
VSEPR Model
• To determine the shape of a molecule, we distinguish
between lone pairs (or non-bonding pairs, those not in a
bond) of electrons and bonding pairs (those found
between two atoms).
• We define the electron domain geometry (or orbital
geometry) by the positions in 3D space of ALL electron
pairs (bonding or non-bonding).
• The electrons adopt an arrangement in space to minimize
e--e- repulsion.
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
• Examples – Draw the Lewis structures,
and then determine the orbital geometry
of each. Indicate the number of electron
domains first:
1. H2S
2. CO2
3. PCl3
4. CH4
5. SO2
Molecular
Geometries
and Bonding
• Examples – Draw the Lewis structures, and
then determine the orbital geometry of each:
•
e- domain
orbital geometry
•
or e- domain geom
1. H2S
4
tetrahedral
2. CO2
2
linear
3. PCl3
4
tetrahedral
4. CH4
4
tetrahedral
5. SO2
3
trigonal planar
Molecular
Geometries
and Bonding
• To determine the electron pair ( electron domain)
geometry:
• draw the Lewis structure,
• count the total number of electron pairs around the central atom,
• arrange the electron pairs in one of the above geometries to
minimize e--e- repulsion, and count multiple bonds as one
bonding pair.
• But then we have to account for the shape of the
molecule
Molecular
Geometries
and Bonding
Molecular Geometries
Within each electron
domain, then, there
might be more than
one molecular
geometry.
Molecular
Geometries
and Bonding
Linear Electron Domain
• In this 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.
Molecular
Geometries
and Bonding
Nonbonding Pairs and Bond Angle
• Nonbonding pairs are physically
larger than bonding pairs.
• Therefore, their repulsions are
greater; this tends to decrease
bond angles in a molecule.
Molecular
Geometries
and Bonding
• By experiment, the H-X-H bond angle decreases on
moving from C to N to O:
H
H C H
H
109.5O
H N H
H
107O
O
H
H
104.5O
• Since electrons in a bond are attracted by two nuclei, they do
not repel as much as lone pairs.
• Therefore, the bond angle decreases as the number of lone pairs
increase.
Molecular
Geometries
and Bonding
• Similarly, electrons in multiple bonds repel more than
electrons in single bonds.
Cl
111.4o
Cl
C O
124.3o
Molecular
Geometries
and Bonding
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.
Molecular
Geometries
and Bonding
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. Molecular
Geometries
and Bonding
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
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
• Examples – Same examples as
before, now determine the the
molecular geometry of each, including
shapes and bond angles:
1. H2S
2. CO2
3. PCl3
4. CH4
5. SO2
Molecular
Geometries
and Bonding
• Examples – Determine the molecular
geometry of each, including shapes and
bond angles:
•
Shape
Angles
1. H2S
bent
<109°
2. CO2
linear
180°
3. PCl3
trigonal pyramid
<109°
4. CH4
tetrahedral
109.5°
5. SO2
bent
<120°
Molecular
Geometries
and Bonding
Molecules with Expanded Valence Shells
• For elements of the 3rd shell and below, some atoms can
have expanded octets.
• AB5 (trigonal bipyramidal) or AB6 (octahedral) electron
pair geometries.
Molecular
Geometries
and Bonding
Trigonal Bipyramidal Electron
Domain (5 e domains)
• There are two
distinct positions in
this geometry:
 Axial
 Equatorial
Molecular
Geometries
and Bonding
Molecules with Expanded Valence Shells
• To minimize e--e- repulsion, lone pairs are always
placed in equatorial positions.
Molecular
Geometries
and Bonding
Trigonal Bipyramidal
(e- domain)
There are four distinct molecular
geometries in this domain:
*Trigonal bipyramidal
*Seesaw
*T-shaped
*Linear
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
Trigonal Bipyramidal Electron
Domain
Lower-energy conformations result from
having nonbonding electron pairs in
equatorial, rather than axial, positions in this
geometry.
Molecular
Geometries
and Bonding
Octahedral electron domain
• 6 electron pairs
All positions are equivalent in the
octahedral domain.
There are three molecular
geometries:
*Octahedral
*Square pyramidal
*Square planar
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
• Examples – Determine the Shape of
each, indicate the electron domain,
molecular geometry and angles.
1. PF5
2. XeF4
3. SF6
4. SCl4
Molecular
Geometries
and Bonding
• Examples – Determine the Shape of
each:
1. PF5
trigonal bipyramid 90°, 120°
2. XeF4 square planar
90°
3. SF6
octahedral
90°
4. SCl4 see-saw
<90°,<
120°
See moving chart
Molecular
Geometries
and Bonding
Larger Molecules
In larger molecules,
it makes more
sense to talk about
the geometry about
a particular atom
rather than the
geometry of the
molecule as a
whole.
Molecular
Geometries
and Bonding
Larger Molecules
This approach
makes sense,
especially because
larger molecules
tend to react at a
particular site in the
molecule.
Molecular
Geometries
and Bonding
Shapes of Larger Molecules
• In acetic acid, CH3COOH, there are three central atoms.
• We assign the geometry about each central atom
separately.
Molecular
Geometries
and Bonding
•Examples – Determine the shape and angles
about each atom:
O
• 1
H
• 2
H
C
C
C
O
H
C
C
N
H
H
H
H
Molecular
Geometries
and Bonding
• Examples – Determine the shape
and angles about each atom:
• 1
Trigonal planar
linear
O
H
• 2
H
C
C
C
H
O
Bent
trigonal pyramid
H
Trigonal planar
C
C
N
H
Molecular
Geometries
and Bonding
H
H
Molecular Shape and
Molecular Polarity
• When there is a difference in electronegativity between
two atoms, then the bond between them is polar.
• It is possible for a molecule to contain polar bonds, but
not be polar.
• For example, the bond dipoles in CO2 cancel each other
because CO2 is linear.
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
• In water, the molecule is not linear and the bond dipoles
do not cancel each other.
• Therefore, water is a polar molecule.
• The overall polarity of a molecule depends on its
molecular geometry.
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
Polarity
By adding the
individual bond
dipoles, one can
determine the
overall dipole
moment for the
molecule.
Molecular
Geometries
and Bonding
Polarity
Molecular
Geometries
and Bonding
To remember
• Trigonal Planar molecular shape if the 3
surroundings atoms are the same the
molecule is non polar molecule because
the dipoles cancel each other.
• Tetrahedral: if 4 surrounding atoms are
the same molecule non-polar. This is
important for carbon compounds with 4
single bonds.
Molecular
Geometries
and Bonding
Examples – Determine whether each is
polar or nonpolar:
1. CCl4
2. PCl3
3. BF3
4. BrClFCH
5. SO3
Molecular
Geometries
and Bonding
Examples – Determine whether each is
polar or nonpolar:
1. CCl4
nonpolar
2. PCl3
polar
3. BF3
nonpolar
4. BrClFCH polar
5. SO3
nonpolar
Molecular
Geometries
and Bonding