Chemistry: A Molecular Approach, 1st Ed.
Nivaldo Tro
Roy Kennedy
Massachusetts Bay Community College
Wellesley Hills, MA
2008, Prentice Hall
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
Tro, Chemistry: A Molecular Approach
2
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
Tro, Chemistry: A Molecular Approach
3
Using Lewis Theory to Predict
Molecular Shapes
 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
Tro, Chemistry: A Molecular Approach
4
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
Tro, Chemistry: A Molecular Approach
5
Tro, Chemistry: A Molecular Approach
6
Electron Groups
 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
••
•O
•
••
N
Tro, Chemistry: A Molecular Approach
••
O ••
••
there are 3 electron groups on N
1 lone pair
1 single bond
1 double bond
7
Molecular Geometries
 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
Tro, Chemistry: A Molecular Approach
8
Linear Geometry
 when there are 2 electron groups around the central
atom, they will occupy positions opposite each other
around the central atom
 this results in the molecule taking a linear geometry
 the bond
angle is 180° 





Cl
Be

Tro, Chemistry: A Molecular Approach
Cl



O

C
O



9
Trigonal Geometry
 when there are 3 electron groups around the central
atom, they will occupy positions in the shape of a
triangle around the central atom
 this results in the molecule taking a trigonal planar
geometry
 the bond angle is 120°




F
B
F






F



Tro, Chemistry: A Molecular Approach
10
Not Quite Perfect Geometry
Because the bonds are not
identical, the observed angles are
slightly different from ideal.
Tro, Chemistry: A Molecular Approach
11
Tetrahedral Geometry
 when there are 4 electron groups around the central
atom, they will occupy positions in the shape of a
tetrahedron around the central atom
 this results in the molecule taking a tetrahedral
geometry
 the bond angle is 109.5°



F






F
C
F






F



Tro, Chemistry: A Molecular Approach
12
Trigonal Bipyramidal Geometry
••
• Cl
• ••
••
• Cl
• ••
••
• Cl•
• • •• •
Cl •
P ••
••
Cl ••
••
Tro, Chemistry: A Molecular Approach
13
Octahedral Geometry
••
•F
• ••
• ••
•F
••
••
•F •
• •
S
• F•
• •• •
••
F ••
••
•• •
F•
••
Tro, Chemistry: A Molecular Approach
14
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
Tro, Chemistry: A Molecular Approach
15
Effect of Lone Pairs
The nonbonding
bonding electrons
electrons
are shared
are localized
by twoon
atoms,
the central
so some
atom,
of the
so
negative
area
of negative
charge ischarge
removed
takesfrom
more
thespace.
central atom.
Tro, Chemistry: A Molecular Approach
16
Derivative of Trigonal Geometry
 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°





O
S
O


Tro, Chemistry: A Molecular Approach





O
S
O







O
S
O


17
Pyramidal Shape
Tro, Chemistry: A Molecular Approach
18
Bond Angle Distortion from Lone Pairs
Tro, Chemistry: A Molecular Approach
19
Tetrahedral-Bent Shape
Tro, Chemistry: A Molecular Approach
20
Bond Angle Distortion from Lone Pairs
Tro, Chemistry: A Molecular Approach
21
Tetrahedral-Bent Shape
 



  

 



  
1
O  Cl  O

Tro, Chemistry: A Molecular Approach
22
Replacing Atoms with Lone Pairs
in the Trigonal Bipyramid System
Tro, Chemistry: A Molecular Approach
23
T-Shape
Tro, Chemistry: A Molecular Approach
24
Linear Shape
Tro, Chemistry: A Molecular Approach
25
Tro, Chemistry: A Molecular Approach
26
Predicting the Shapes
Around Central Atoms
Draw the Lewis Structure
Determine the Number of Electron Groups around the
Central Atom
Classify Each Electron Group as Bonding or Lone pair,
and Count each type
1)
2)
3)

4)
remember, multiple bonds count as 1 group
Use Table 10.1 to Determine the Shape and Bond Angles
Tro, Chemistry: A Molecular Approach
27
Representing 3-Dimensional Shapes on a
2-Dimensional Surface
 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 hashed wedge
Tro, Chemistry: A Molecular Approach
28
Multiple
Central
Atoms
 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

shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
Tro, Chemistry: A Molecular Approach
H O 
|
||  
HCCOH
|

H
29
Describing the Geometry
of Methanol
Tro, Chemistry: A Molecular Approach
30
Describing the Geometry
of Glycine
Tro, Chemistry: A Molecular Approach
31
Polarity of Molecules

in order for a molecule to be polar it must
1)
have polar bonds


2)
have an unsymmetrical shape


vector addition
polarity affects the intermolecular forces of attraction

therefore boiling points and solubilities


electronegativity difference - theory
bond dipole moments - measured
like dissolves like
nonbonding pairs affect molecular polarity, strong pull
in its direction
Tro, Chemistry: A Molecular Approach
32
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.
Tro, Chemistry: A Molecular Approach
33
Tro, Chemistry: A Molecular Approach
34
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.
Tro, Chemistry: A Molecular Approach
35
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.
Tro, Chemistry: A Molecular Approach
36
Molecule Polarity
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.
Tro, Chemistry: A Molecular Approach
37
Molecular Polarity Affects
Solubility in Water
 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
Tro, Chemistry: A Molecular Approach
38
Problems with Lewis Theory
 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
Tro, Chemistry: A Molecular Approach
39