Download Lecture #21 Shapes of Molecules

Chapter 9
Molecular Geometry
Lewis Theory-VSEPR
Valence Bond Theory
Molecular Orbital Theory
Lewis Structures and the
“Real” 3D-Shape of Molecules
Sulfanilamide
Lewis Theory
of
Molecular Shape and Polarity
Structure Determines Properties!
Properties of molecular substances depend on the
structure of the molecule.
The structure includes many factors, such as:
Skeletal arrangement of the atoms
Kind of bonding between the atoms
Shape of the molecule
Molecular Geometry
We can describe the shape of a molecule with terms that relate to
geometric figures
These geometric figures have characteristic “corners”
(indicating the positions of atoms)
The geometric figures also have characteristic angles that we call
bond angles.
Lewis Theory Predicts
“Electron Groups”
“Electron groups” - regions of electrons around an atom
Regions result from placing shared pairs of
valence electrons between bonding nuclei
Regions result from placing unshared
valence electrons on a single nuclei
Lewis Theory of Molecular Shapes
Electron groups repel each other.
Predicting the shapes of molecules
1) The arrangement of the electron groups will be
determined by trying to minimize repulsions between them.
2) The arrangement of atoms (“molecular shape”)
surrounding a central atom will be determined by where the bonding
electron groups are.
3) “1” and “2” are not necessarily the same
VSEPR Theory
Valence shell electron pair repulsion (VSEPR) Electron groups around the central atom will be most
stable when they are as far apart as possible.
Electron Groups
A Lewis structure predicts the number of valence electron
pairs around a central atom(s).
Each lone pair of electrons or odd electron constitutes one
electron group on a central atom.
Each bond constitutes one electron group, regardless of
whether it is single, double, or triple
O
N
O
O
N
O
There are three electron groups
around N
There are three electron groups
around N
“steric number = 3”
one lone pair
one single bond
one double bond
one odd electron
one single bond
one double bond
Electron Group Geometry
There are five basic arrangements of electron groups
around a central atom.
For molecules that exhibit resonance, it doesn’t matter
which resonance form you use – the electron group
geometry will be the same.
linear
triangular
tetrahedral
trigonal
bipyramidal
octahedral
Linear Electron Geometry
When there are two electron groups around the central atom,
they will occupy positions on opposite sides of the central atom.
This results in the electron groups taking a linear geometry.
The bond angle is 180°.
Cl
Be
Cl
O
C
O
Trigonal Planar Electron Geometry
When there are three electron groups around the central atom, they
will occupy positions in the shape of a triangle around the central atom.
This results in the electron groups in a trigonal planar geometry.
The bond angle is 120°
F
Be
F
F
Tetrahedral Electron Geometry
When there are four electron groups around the central atom, they
will occupy positions in the shape of a tetrahedron around the central
atom.
This results in the electron groups taking a tetrahedral geometry.
The bond angle is 109.5°
F
F
C
F
F
Trigonal Bipyramidal Electron Geometry
When there are five electron groups around the central atom, they will
occupy positions in the shape of two tetrahedra that are base-to-base with the
central atom in the center of the shared bases.
This results in the electron groups in a trigonal bipyramidal geometry.
Trigonal Bipyramidal Electron Geometry
The positions above and below the central
atom are called the axial positions.
The positions in the same base plane as
the central atom are called the
equatorial positions.
The bond angle between equatorial
positions is 120°.
The bond angle between axial and
equatorial positions is 90°.
Octahedral Electron Geometry
When there are six electron groups around the central atom, they will
occupy positions in the shape of two square-base pyramids that are baseto-base with the central atom in the center of the shared bases
This results in the electron groups taking an octahedral geometry.
All positions around the central atom are equivalent.
The bond angle is 90°
Molecular Geometry
1) The actual geometry (“molecular geometry”) of a
molecule may be different from the electron geometry.
2) When the electron groups are attached to atoms of
different size, or when the bonding to one atom is
different than the bonding to another, this will affect
the molecular geometry around the central atom.
3) Lone pairs occupy space on the central atom, but are
not “seen” as points on the molecular geometry.
Not Quite Perfect Geometry
O
H
C
H
Because the bonds and atom sizes are not
identical in formaldehyde, the observed
angles are slightly different from ideal.
The Effect of Lone Pairs
The bonding electrons
are shared by two
atoms, so some of the
negative charge is
removed from the
central atom.
The nonbonding
electrons are localized
on the central atom, so
area of negative charge
takes more space.
The Effect of Lone Pairs
Lone pair groups “occupy more space” on the central atom
than bonding electrons.
Relative sizes of repulsive force interactions:
Lone Pair – Lone Pair >
Lone Pair – Bonding Pair >
Bonding Pair – Bonding Pair
This affects the bond angles, making the bonding pair –
bonding pair angles smaller than expected.
Bond Angle Distortion from Lone Pairs
Molecular geometries derived from
tetrahedral electron geometry.
VSEPR Theory
Bond Angle Distortion from Lone Pairs
Tetrahedral
molecular
shape
Pyramidal
molecular
shape
Bent
molecular
shape
Bent or Angular Molecular Geometry:
a Derivative of Trigonal Planar Electron Geometry
When there are three electron groups around the central
atom, and one of them is a lone pair, the resulting shape
of the molecule is called a angular or bent shape.
The bond angle is less than 120°.
O
SO2
S
O
Bent Molecular Geometry
O
Cl
O
110 º
ClO2
Trigonal Bipyramidal Electron Geometry
Molecular geometries derived from trigonal
bipyramidal electron geometry.
See Saw Moleclular Geometry
a Derivatives of Trigonal Bipyramidal Electron Geometry
When there are five electron groups around the central atom, and some are lone
pairs, the lone pairs will occupy the equatorial positions because there is more room .
F
F
S
F
SF4
F
See Saw Moleclular Geometry
a Derivatives of Trigonal Bipyramidal Electron Geometry
When there are five electron groups around the central atom, and one is a
lone pair, the result is called the seesaw shape.
F
F
S
F
SF4
F
T-Shaped Molecular Geometry
a Derivative of Trigonal Bipyramidal Electron Geometry
When there are five electron groups around the central atom, and
two are lone pairs, the result is called the T-shaped.
BrF3
Linear Molecular Geometry
a Derivatives of Trigonal Bipyramidal Electron Geometry
When there are five electron groups around the central atom, and
three are lone pairs, the result is a linear shape .
XeF2
Molecular geometries derived from
octahedral electron geometry.
Square Pyramidal Molecular Geometry
a Derivatives of Octahedral Electron Geometry
BrF5
When there are six electron groups around the central atom, and
one is a lone pair, the result is called a square pyramid shape.
The bond angles between axial and equatorial positions is less than 90°
Square Planar Molecular Geometry
a Derivatives of Octahedral Electron Geometry
XeF4
When there are six electron groups around the central atom, and
two are lone pairs, the result is called a square planar shape.
The bond angles between equatorial positions is 90°.
Predicting the Shapes Around Central Atoms
1. Draw the Lewis structure
2. Determine the number of electron groups around
the central atom
3. Classify each electron group as bonding or lone
pair, and count each type
4. Determine the shape and bond angles
Predict the geometry and bond angles of PCl3
1. Draw the Lewis structure
Cl
26 valence electrons
2. Determine the number of
electron groups around
central atom
four electron groups around P
3. Classify the electron groups
a) three bonding groups
b) one lone pair
Cl
P
Cl
Predict the geometry and bond angles of PCl3
4. Determine the shape and bond angles
a) four electron groups around P =
tetrahedral electron geometry
b) three bonding + one lone pair = trigonal
pyramidal molecular geometry
c) trigonal pyramidal = bond angles less
than 109.5°
Predict the molecular geometry and bond angles in SiF5─
1. Draw the Lewis structure
40 valence electrons
2. Determine the number of electron
groups around central atom
five electron groups around Si
3. Classify the electron groups
a) five bonding groups
b) 0 lone pairs
F
F
F
Si
F
F
Predict the molecular geometry and bond angles in SiF5─
4. Determine the shape and bond angles
a) five electron groups around Si = trigonal
bipyramidal electron geometry
b) five bonding + 0 lone pairs = trigonal
bipyramidal molecular geometry
c) trigonal bipyramidal = bond angles of than
120° (eq-eq) and 90º (ax-eq)
Predict the molecular geometry and bond angles in ClO2F
1. Draw the Lewis structure
26 valence electrons
2. Determine the number of electron
groups around central atom
4 electron groups around Cl
3. Classify the electron groups
a) three bonding groups
b) one lone pair
Predict the molecular geometry and bond angles in ClO2F
4. Determine the shape and bond angles
a) four electron groups around Cl =
tetrahedral electron geometry
b) 3 bonding + 1 lone pair = trigonal
pyramidal molecular geometry
c) trigonal pyramidal = bond angles of
<109.5°
Molecules with Multiple Central Atoms
Methanol
H
H
O
N
C
C
H
H
Glycine
O
H
Polarity of Molecules
Polarity of Molecules
For a molecule to be polar, it must
have polar bonds, and
have an unsymmetrical shape
Polarity affects the intermolecular forces of attraction
and therefore affects boiling points and solubilities
Nonbonding pairs affect molecular polarity.
When describing the polarity of a molecule, we must
consider bond polarities as
VECTOR QUANTITIES
quantities with magnitude and direction.
Common Cases of Adding Dipole Moments to Determine
Whether a Molecule is Polar
Molecular Polarity
The O─C bond is polar. The bonding
electrons are pulled equally toward both O
ends of the molecule. The net result is a
nonpolar molecule.
Molecular Polarity
The H─O bond is polar. Both sets of bonding
electrons are pulled toward the O end of the
molecule. The net result is a polar molecule.
Predicting Polarity of Molecules
1. Draw the Lewis structure and determine the
molecular geometry.
2. Determine whether the bonds in the molecule
are polar.
3. Determine whether the polar bonds add
together to give a net dipole moment.
Predict whether NH3 is a polar molecule
1. Draw the Lewis
structure and determine
the molecular geometry
a) eight valence electrons
b) three bonding + one lone
pair = trigonal pyramidal
molecular geometry
Predict whether NH3 is a polar molecule
2. Determine if the bonds are polar
a) electronegativity difference
b) if the bonds are not polar, we can
stop here and declare the molecule
will be nonpolar
ENN = 3.0
ENH = 2.1
3.0 − 2.1 = 0.9
therefore the bonds
are polar covalent
Predict whether NH3 is a polar molecule
3) Determine whether the polar
bonds add together to give a
net dipole moment
a) vector addition
b) generally, asymmetric shapes
result in uncompensated
polarities and a net dipole
moment
The H─N bond is polar. All
the sets of bonding
electrons are pulled toward
the N end of the molecule.
The net result is a polar
molecule.
Decide whether the following molecule is polar
EN
O = 3.5
N = 3.0
Cl = 3.0
S = 2.5
Trigonal
Bent
1. polar bonds, N-O
2. asymmetrical shape
polar
Decide whether the following molecule is polar
Trigonal
Planar
EN
O = 3.5
S = 2.5
1. polar bonds, all S-O
2. symmetrical shape
nonpolar
What about Tetrahedral Geometry ?
Some molecules are inherently polar because of the atoms which
they contain and the arrangement of these atoms in space.
H2 O
δ−
NH3
δ+
CH2O
HCl
A crude representation
of a polar molecule
Other molecules are considered nonpolar
CH4
BH3
C 2 H2
Nonpolarized
electron
clouds
CO2
Molecular Formula
➡
Structural Formula
➡
Dot Diagram
➡
Molecular Shape
➡
Molecular Polarity
Intermolecular Forces
Melting Point, Boiling Point, Solubility