Lecture 10

Report
Lecture 10: Covalent Bonding Pt 2:
VSEPR Theory (Ch 8)
Suggested HW: (Ch 8) 19, 23(a, c and d only), 28, 29, 34
* Bond angles are not required. Label the geometries of each
molecule. Label molecules as either polar or nonpolar.
Introduction
 To date, we have learned about the Lewis structures of covalent bonds
 Lewis structures give insight into how atoms are bonded within a
molecule, but does NOT tell us about the shape (molecular geometry),
of the molecule
 Molecular geometry plays a major role in the properties of a
substance.
Considering Molecular Geometry
 If we draw the Lewis structures of water without considering geometry, we
would derive the following:
δ+
H
δ-
O
δ+
H
• The Lewis structure suggests that water is a linear (straight) molecule.
• However, if this were true, then the dipoles moments would be in
opposite directions, as described above, and water would be a
nonpolar molecule
 If this were the case, life as we know it would be very different
Considering Molecular Geometry
 The actual geometry of water is shown below:
δδ+
H
O
104.5o
+
H
=
δ+
• This is a bent geometry. The angle between the atoms is 104.5o. In
this geometry, the molecule has a net dipole moment directed
upward, which is why water is polar.
• How do we determine the geometry?
VSEPR Theory
• The images below show balloons tied together at their ends.
• There is an optimum geometry for each number of balloons, and the balloons
spontaneously attain the lowest-energy arrangement.
• In other words, the balloons try to “get out of each other’s way” as best they
can. These arrangements maximize the distance between the balloon centers.
Electrons behave the same exact way.
VSEPR
• In the valence-shell electron-pair repulsion theory (VSEPR), the electron
domains around a central atom:
– are arranged as far apart from each other as possible
– have the least amount of repulsion of the negatively
charged electrons
– have a geometry around the central atom that determines molecular
shape
Using VSEPR To Predict Geometry
STEP 1
Figure out the Lewis dot structure of
the molecule.
Determining Polarity
 Now that we know the geometry of molecules, we can determine whether
or not the molecule is polar (has an overall dipole moment)
• A polar molecule
– contains polar bonds, as determined from differences in
electronegativity (lecture 14)
– has a separation of positive and negative partial charges, called
a dipole, indicated with + and –
– has dipoles that do not cancel (not symmetrical)
Nonpolar Molecules
• A nonpolar molecule
– contains nonpolar bonds, as determined from differences in
electronegativity
– Or may be symmetrical
– Or dipoles cancel
Cl
+
δ
H
Cl
HH
Hδ+
δC
H
+
δ
δ
-O
δ
+
C
O
δ
OVERALL DIPOLE = 0
H
+
δ
Symmetrical
Total
Electron
Domains
2
Domain Geometry
Bonding Lone Pair
Around
Domains Domains
Central Atom
2
MOLECULAR
GEOMETRY
A
B
0
B
Linear
Ex. CO2
B
3
0
Trigonal
planar
A
B
3
B
Ex. BH3
Bent
104.5o
••
A
2
1
B
B
Ex. NO2-
Just a note
 Any molecule containing only two atoms must be linear. There is no
other possible arrangement. Ex. H2, HCl, CO, etc.
Examples
 Give the Lewis structures and geometries of the following
molecules. Label each as polar or nonpolar:
 SO3
 SO2
 F2
4-Coordinate Molecules Have a Tetrahedral Arrangement
• A tetrahedron is a shape
consisting of 4 triangular faces.
The vertices are separated by an
angle of 109.5o, and each
position is equivalent.
• Another way to view a
tetrahedron is to imagine a cube
with atoms at opposite corners,
with the central atom at the
center of the cube.
Total
Electron
Domains
Domain Geometry
Bonding Lone Pair
Around
Domains Domains
Central Atom
MOLECULAR
GEOMETRY
B
4
0
Tetrahedral
A
B
B
B
Ex. CH4
••
4
3
1
A
B
Trigonal
Pyramidal
B
B
Ex. NH3
••
Bent
A
104.5o
B
2
2
B
Ex. H2O
Examples
 Give the chemical structures and geometries of the
following molecules. Label each as polar or nonpolar.
 SO42 PF3
 OF2
 CCl4
Expanded Electron Domains
 As stated in the previous lecture, central atoms with a principal
quantum number of n>3 can accommodate more than 8 valence
electrons.
 In many instances, there will be 5 or 6 bonds around these central
atoms
 The regions occupied by the constituent atoms in a 5-coordinate
structure are not equivalent. The constituent atoms may be either
equatorial or axial.
Five-Coordinate Molecules
Z
X
Axial position (z axis)
Equatorial position (x-y plane)
Y
• Five coordinate molecules assume
some variation of the trigonal
bipyramidal configuration shown
to the right.
• If you have a 5 coordinate molecule which contains a
lone pair, like SF4, the lone pair will go in an equatorial
position.
LONE PAIR WANT TO BE AS FAR AWAY FROM OTHER
ELECTRON DOMAINS AS POSSIBLE, AND SHOULD
ALWAYS BE PLACED IN EQUITORIAL POSITIONS !!!!
Total
Electron
Domains
Domain Geometry
Bonding Lone Pair
Around
Domains Domains
Central Atom
MOLECULAR
GEOMETRY
B
5
0
B
B
A
B
B
5
4
1
B
B
2
B
Ex. PCl5
Seesaw
A
B
3
Trigonal
Bipyramidal
A
B
B
Ex. SF4
T-shaped
B
Ex. ClF3
A Five-Coordinate molecule with 3 Lone pairs is
LINEAR
Symmetrical about the central atom.
Ex. XeF2
Note: In this chapter, you will find
examples of noble gases acting as
central atoms.
Examples
 Give the chemical structures and geometries of the
following molecules. Label each as polar or nonpolar.
 PBr5
 TeCl4
 IOF2-
Six-Coordinate Molecules Take on an Octahedral
Geometry
 Unlike a trigonal bipyramid, the equatorial and axial
positions in an octahedral are equivalent.
 When placing lone pairs in the structure, we must still
maximize their distance. It is customary to first place
lone pair in the axial positions.
Lone Pairs Migrate As Far Away From One Another As
Possible
First electron pair is
placed in an axial
position
If a second lone pair
exists, it is placed the
maximum distance
(180o) from the 1st pair
Total
Electron
Domains
Domain Geometry
Bonding Lone Pair
Around
Domains Domains
Central Atom
MOLECULAR
GEOMETRY
Octahedral
B
6
0
B
B
B
A
B
B
Ex. SF6
6
Square
pyramidal
5
1
••
B
B
B
A
B
B
Ex. BrF5
Total
Electron
Domains
Domain Geometry
Bonding Lone Pair
Around
Domains Domains
Central Atom
MOLECULAR
GEOMETRY
Square Planar
••
6
4
2
B
B
B
A
B
••
Ex. XeF4
Examples
• Give the chemical structures and geometries of the following
molecules. Label each as polar or nonpolar.
– SeF6
– ICl5
– XeF4

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