Transcript Chapter 10-part 1

Chapter 10
Chemical
Bonding II
Structure Determines Properties!
properties of molecular substances depend on the structure
of the molecule
 the structure includes many factors, including:
◦ the skeletal arrangement of the atoms
◦ the kind of bonding between the atoms
 ionic, polar covalent, or covalent
◦ the shape of the molecule
 bonding theory should allow you to predict the shapes of
molecules
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Molecular Geometry
Molecules are 3-dimensional objects
 We often describe the shape of a molecule with terms that
relate to geometric figures
 These geometric figures have characteristic “corners” that
indicate the positions of the surrounding atoms around a
central atom in the center of the geometric figure
 The geometric figures also have characteristic angles that we
call bond angles
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Using Lewis Theory to Predict
Molecular Shapes
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Lewis theory predicts there are regions of electrons in an
atom based on placing shared pairs of valence electrons
between bonding nuclei and unshared valence electrons
located on single nuclei
this idea can then be extended to predict the shapes of
molecules by realizing these regions are all negatively charged
and should repel
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VSEPR Theory
electron groups around the
central atom will be most
stable when they are as far
apart as possible – we call this
valence shell electron pair
repulsion theory
◦ since electrons are
negatively charged, they
should be most stable when
they are separated as much
as possible
 the resulting geometric
arrangement will allow us to
predict the shapes and bond
angles in the molecule
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Electron Groups
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the Lewis structure predicts the arrangement of valence
electrons around the central atom(s)
each lone pair of electrons constitutes one electron group on
a central atom
each bond constitutes one electron group on a central atom
◦ regardless of whether it is single, double, or triple
there are 3 electron groups on N
1 lone pair
1 single bond
1 double bond
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Molecular Geometries
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there are 5 basic arrangements of electron groups around a
central atom
◦ based on a maximum of 6 bonding electron groups
 though there may be more than 6 on very large atoms,
it is very rare
each of these 5 basic arrangements results in 5 different
basic molecular shapes
◦ in order for the molecular shape and bond angles to be a
“perfect” geometric figure, all the electron groups must
be bonds and all the bonds must be equivalent
for molecules that exhibit resonance, it doesn’t matter which
resonance form you use – the molecular geometry will be
the same
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Parent electronic structure
Examples

How many electron groups (charge clouds) are around the central atom in
the following?
SO2
NH4+
PCl5
Trigonal Bipyramidal
Geometry
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when there are 5 electron groups around the
central atom, they will occupy positions in the
shape of a two tetrahedral that are base-to-base
with the central atom in the center of the shared
bases
this results in the molecule taking a trigonal
bipyramidal 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°
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Octahedral Geometry
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when there are 6 electron groups around the central atom, they will
occupy positions in the shape of two square-base pyramids that are
base-to-base with the central atom in the center of the shared bases
this results in the molecule taking an octahedral geometry
◦ it is called octahedral because the geometric figure has 8 sides
all positions are equivalent
the bond angle is 90°
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The Effect of Lone Pairs
lone pair groups “occupy more space” on the central atom
◦ because their electron density is exclusively on the central atom rather
than shared like bonding electron groups
 relative sizes of repulsive force interactions is:
Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair –
Bonding Pair
 this effects the bond angles, making them smaller than expected
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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
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Derivative of Trigonal Geometry
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when there are 3 electron groups around the central atom, and 1 of
them is a lone pair, the resulting shape of the molecule is called a
trigonal planar - bent shape
the bond angle is < 120°
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Bond Angle Distortion from Lone
Pairs
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Replacing Atoms with Lone Pairs
in the Trigonal Bipyramid System
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T-Shape
Linear Shape
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Predicting the Shapes Around Central Atoms
Total # of “Parent” electronic
e- groups
geometry
on central
atom
2
3
3
4
4
4
5
Linear
Trigonal Planar
Trigonal Planar
Tetrahedral
Tetrahedral
Tetrahedral
Trigonal Bipyramidal
#
Bond
ed
atom
s
2
3
2
4
3
2
5
5
Trigonal Bipyramidal
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1
Linear
Trigonal Planar
Bent
Tetrahedral
Trigonal Pyramidal
Bent
Trigonal
Bipyramidal
Seesaw
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5
6
6
6
Trigonal Bipyramidal
Trigonal Bipyramidal
Octahedral
Octahedral
Octahedral
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2
3
0
1
2
T-shaped
Linear
Octahedral
Square Pyramidal
Square Planar
2
6
5
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#
Lone
pairs
Idealized
molecular shape
Idealized
bond
angles
0
0
1
0
1
2
0
180o
120 o
120 o
109.5 o
109.5 o
109.5 o
90 o, 120 o,
180 o
90 o, 120 o,
180 o
90 o, 180 o
180 o
90 o, 180 o
90 o, 180 o
90 o, 180 o
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Real bond angles vs. Idealized bond
angles
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VSEPR predicts the idealized bond angle(s) by assuming that all electron
groups take up the same amount of space. Since lone pairs are attracted to
only one nucleus, they expand into space further than bonding pairs, which
are attracted to two nuclei. As a result, real molecules that has lone pairs on
the central atom often have bond angles that are slightly different than the
idealized prediction
Central atom without lone pairs has the
same real bond angle as the idealized
angle.
The exceptions to this are square planar
shapes and linear (derived from trigonal
bipyramidal electronic structure) shapes
where the lone pairs offset one another,
thus causing no deviation from ideality.
Example
Lewis structure
Shape
Idealized bond
angle
Real bond angle
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Multiple Central Atoms
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many molecules have larger structures with many interior atoms
we can think of them as having multiple central atoms
when this occurs, we describe the shape around each central atom in
sequence
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Representing 3-Dimensional Shapes
on a 2-Dimensional Surface
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one of the problems with drawing molecules is trying to show their
dimensionality
by convention, the central atom is put in the plane of the paper
put as many other atoms as possible in the same plane and indicate with a
straight line
for atoms in front of the plane, use a solid wedge
for atoms behind the plane, use a dashed wedge
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Polarity of Molecules
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in order for a molecule to be polar it must
1) have polar bonds
 electronegativity difference - theory
 bond dipole moments - measured
2) have an unsymmetrical shape
 vector addition
polarity affects the intermolecular forces of attraction
◦ therefore boiling points and solubilities
 like dissolves like
nonbonding pairs affect molecular polarity, strong pull in its direction
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Molecule Polarity
The H-Cl bond is polar. The bonding electrons are
pulled toward the Cl end of the molecule. The net result
is a polar molecule.
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Molecule 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.
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Molecule Polarity
The H-O bond is polar. The both sets of bonding electrons
are pulled toward the O end of the molecule. The net result
is a polar molecule.
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Factors Affecting Dipole
Moments
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Lone-pair electrons on oxygen and nitrogen project out into space
away from positively charged nuclei giving rise to a considerable
charge separation and contributing to the dipole moment
Molecular Polarity Affects
Solubility in Water
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polar molecules are attracted to other polar
molecules
since water is a polar molecule, other polar
molecules dissolve well in water
◦ and ionic compounds as well
some molecules have both polar and nonpolar
parts
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A Soap Molecule
Sodium Stearate
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Example - Decide Whether the Following
Are Polar
EN
O = 3.5
N = 3.0
Cl = 3.0
S = 2.5
••
•O
•
••
••
•O
•
••
•O•
• •
S
••
O ••
••
N
••
Cl ••
••
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Problems with Lewis Theory
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Lewis theory gives good first approximations of the bond
angles in molecules, but usually cannot be used to get the
actual angle
Lewis theory cannot write one correct structure for many
molecules where resonance is important
Lewis theory often does not predict the correct magnetic
behavior of molecules
◦ e.g., O2 is paramagnetic, though the Lewis structure
predicts it is diamagnetic
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