Transcript Chapter 10 - Chemistry

Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond
Theory, and Molecular Orbital Theory
10.1 Artificial Sweeteners: Fooled by Molecular Shape (Suggested
Reading)
10.2 VSEPR Theory: The Five Basic Shapes [10.1]
10.3 VSEPR Theory: The Effect of Lone Pairs [10.1]
10.4 VSEPR Theory: Predicting Molecular Geometries [10.1]
10.5 Molecular Shape and Polarity [10.2]
10.6 Valence Bond Theory: Orbital Overlap as a Chemical Bond [10.3 &
10.4]
10.7 Valence Bond Theory: Hybridization of Atomic Orbitals [10.3 &
10.4]
Chemistry 1011 Y8Y,U
Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Lewis dot structures
Lewis dot structures only give an idea of
the electron distribution in the species.
There is NO INFORMATION about the
molecular geometry, which depends on
the relative position of nuclei around the
central atom.
?
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
VSEPR Model
One may connect the information of electron
distribution in a Lewis dot structure to
molecular geometry by using the
Valence-Shell Electron-Pair
Repulsion (VSEPR) theory.
The essence of the VSEPR theory:
GROUPS of electrons repel each other,
ending up as far from each other as possible.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Some textbooks talk about
repulsion of ELECTRON PAIRS.
The term
repulsion of ELECTRON GROUPS
is perhaps better, because multiple bonds
are treated the same way as ONE PAIR of
electrons in VSEPR theory even though in
a multiple bond there are more than one
pair of electrons present.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
.
Considering directions around a central
atom,
A lone pair is ONE GROUP of electrons
A single bond is ONE GROUP of electrons
A double bond is ONE GROUP of electrons
A triple bond is ONE GROUP of electrons
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Electron distribution vs. geometry
Electron
distribution
Molecular geometry
The “shape” of
electron group
distribution
The “shape” of nuclear
positions around the central
atom
Lone pairs influence
The “shape” of electron molecular geometry, but they
distribution INCLUDES are not part of this “shape”,
since there are no terminal
all lone pairs
nuclei on lone pairs
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Electron distribution vs. geometry
For simple molecules with a
central atom:
If the central atom has NO lone
pairs on it, then
the electron group distribution
and the molecular geometry
ARE THE SAME!
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Figure of shapes (GROUPS)
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
AXn notation
The central atom A is bonded to n atoms or
functional groups, denoted as X.
This notation ignores lone pairs, so it is
suited for categorizing molecular
geometries, which also ignore lone pairs.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Figure of shapes (2 to 4 GROUPS)
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Figure of shapes (5 GROUPS)
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Figure of shapes (6 GROUPS)
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Comment
With experience,
we tend to start
drawing Lewis dot
structures
with molecular
geometry
information
included…
instead of
instead of
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Getting geometry information
1. Draw the Lewis dot structure
2. Determine the number of electron groups
on the central atom to get electron
group arrangement
3. Use the number of lone pairs and the
arrangement to determine the molecular
geometry
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Advanced geometry considerations
Lone pairs are the “biggest” electron groups
(best at repelling other electron groups).
Triple bonds are the next “biggest” groups.
Double bonds are “smaller”.
Single bonds are the “smallest” electron groups.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Tetrahedral arrangement (advanced)
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Trigonal planar arrangement (advanced)
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Problem
.
What is the arrangement of electron
groups, and geometry around the central
atom for the following molecules?
SF2
+
H3 O
XeO4
AsF5
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Molecular dipole moments
If there are polar covalent bonds (partial
charge separations) in a molecule, the molecule
MAY OR MAY NOT have a permanent dipole
moment.
A permanent dipole moment means there are
regions of the entire molecule that are
permanently partially negative and
permanently partially positive.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Permanent dipole moments
To determine if a molecule has
a permanent dipole moment,
we add together the vectors
that describe the
charge separation of polar
covalent bonds.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Permanent dipole moments
To add vectors, we chain vectors by putting the tail
of the next vector on the head of the previous
vector.
The resultant vector is then drawn from the tail of
the first vector to the head of the last vector in the
chain.
This resultant vector is the permanent dipole
moment.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Recall HCl
.

H - Cl :

δ δ

H - Cl :

We saw earlier that the diatomic
molecule HCl has a polar
covalent bond.
Since there is only one bond, this
one vector of charge separation
ALSO describes the permanent
dipole moment of HCl.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Water
.
The permanent dipole moment in water
can be seen by adding together the charge
separation vectors of the two polar
covalent O-H bonds.
Lewis structure
Adding vectors
Permanent
dipole moment
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Symmetry and dipole moments
A molecule with more than one polar bond
MIGHT NOT have a permanent dipole
moment when the charge separations are
symmetrically distributed so that the
resultant vector sums up to to zero.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Geometry and dipole moments
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Permanent dipole moments and molecular properties
.
Ionic bonds are generally strong because of the
strong electrostatic attraction between
positive and negative charges.
Molecules with permanent dipole moments have regions
with partial positive and negative charges that attract
the opposite regions on other molecules of the same
type.
Such intermolecular forces affect the bulk properties
of collections of molecules.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Quantifying dipole moments

H - Cl :

Dipole
moment for
HCl is 1.08 D
Dipole moments
measure the amount of
charge separation (in
Coulombs) that occurs
over the bond length (in
meters) in a derived unit
called a debye (D)
1 D = 3.34 x 10-30 Cm
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Problem
a) The molecule BrF3 has a dipole
moment of 1.19 D. Which of the
following geometries are possible:
trigonal planar, trigonal pyramidal, or
T-shaped?
b) The molecule TeCl4 has a dipole
moment of 2.54 D. Is the geometry
tetrahedral, seesaw, or square planar?
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Valence bond theory
.
Bonds form between atoms when:
1. Orbitals (the “allowed” electron
distributions) in the atoms overlap
to create molecular bonding orbitals.
2.
Each molecular bonding orbital has
NO MORE THAN 2 electrons in it.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Bond strength
Covalent bonds are strongest when there is maximum orbital
overlap between atomic orbitals. This maximum overlap occurs
in the same direction as the atomic orbitals point.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Hybrids
Hybrids occur when we mix two or more
different types of things from the same
class.
The resultant hybrid shows similarities
to the original things, but is distinctly
different from them.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Hybrid orbitals
The atomic orbitals of atoms can be
mixed together (WHEN REQUIRED!)
to form hybrid atomic orbitals
that are different from the source orbitals.
Such hybrid orbitals are used to better
explain molecular geometry and bonding.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Oxygen atom orbital diagram
.
We would expect water to have a 90 angle between its bonds, based on the
atomic orbitals on oxygen.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Oxygen hybrid orbitals
.
plus
gives
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Oxygen hybrid orbitals
.
plus
gives
One s and
three p orbitals
combine to
give four sp3
hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
In general:
.
A total of n atomic orbitals combine to give n hybrid
orbitals of a given kind.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Hybrid orbitals
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Oxygen hybrid orbital diagram
.
We would expect water to have a ~109.5 angle based on the
hybrid sp3 orbitals on oxygen.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
Determining hybrid orbitals diagrams
.
1. Draw the Lewis dot structure
2. Use VSEPR theory to predict electron
group arrangement
3. Use Table 10.2 to determine what hybrid
orbitals have the same arrangement
4. Create the hybrid orbital diagram based on
changing the ground state diagram
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Problem
Describe the bonding of I3in terms of valence bond theory.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Multiple bonding
Multiple bonds (double or triple bonds) are possible
when more than one set of orbitals can overlap
between two atoms.
The first bond is the sigma (s) bond, which occurs
from orbital overlap on the axis between the atoms.
The second and third bonds are pi (p) bonds that
occur from orbital overlap both above and below
the axis between the two atoms.
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Ethene has a double bond
No orbital overlap between these p
orbitals
Notice we’ve chosen to create
sp2 hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
.
Chemical Bonding II
Ethyne has a triple bond
Notice we’ve
chosen to create sp
hybrid orbitals and
not sp3 or sp2
Molecular Geometry and Chemical Bonding, Paul G. Mezey
Chapter 10
Chemical Bonding II
.
Molecular Geometry and Chemical Bonding, Paul G. Mezey