1
Shape of Molecules
and their Interactions
Chapter 3
Lewis Structures
• Predicts what molecules look like in three dimension.
• The 3-Dimensional structure determines the physical
properties such as B.P., M.P., miscibility, vapor
pressure, heat of evaporation, etc.
• Determined by the number of atoms and lone pairs
connected to the central atom but not the number of
bonds.
• Atoms want to be as far away from each other as possible
to minimize repulsion. (VSEPR)
2
Electronic Geometries
Number of Atoms
Connected
Shape
Geometry
2
AX2
Linear
3
AX3
Trigonal
Planar
4
AX4
Tetrahedral
3
Electronic Geometry
• SO2
• SO3 -2
Shapes of Molecules - Based on what the bonds look like.
Cover the lone pairs and look at the bonds.
4
Electronic Geometry
• SO2
• SO3 -2
Shapes of Molecules - Based on what the bonds look like.
Cover the lone pairs and look at the bonds.
5
Molecular Geometries (Shape)
Number of Atoms
and Lone Pairs
Connected
Shape
Geometry
3
AX3
Trigonal
Planar
3
AX2E
Bent
Molecular Geometries (Shape)
Number of Atoms
and Lone Pairs
4
AX4
4
AX3E
4
AX2E2
Shape
Geometry
Tetrahedral
109.5 °
Trigonal
Pyramidal
107 °
Bent
105°
7
Shape of Larger Molecules
• For the molecule below predict the electronic
geometry and the molecular geometry (Shape)
for each central atom
9
Predicting Type of Covalent Bond
When the sharing of electrons is equal the
bond is considered non polar covalent.
• Unequal sharing of electrons generates a
polar covalent bond. (Dipole)
• Polar covalent bonds might result in polar
molecules.
Electronegativity
• Tendency of some atoms to pull electrons toward
themselves.
• Right side of the periodic table will have the largest
electronegativity.
• Top and right side contains the most selfish
electron huggers.
• F, O, and N are the three most electronegative
elements.
10
Electronegative
Polar Covalent Bond
• Atoms with different
electronegativities result in
a polar covalent bond.
• C-H is considered a non
polar bond
• In a polar covalent
bond, the unequal sharing
of electrons gives the bond
a partially positive and
partially negative end
(dipole).
Think of a magnet
12
Polar Molecules
• Polarity in a molecule depends on its electronic
geometry.
• Perfect geometries are non polar.
• Presence of one lone pair around the central atom
makes the molecule polar.
•
CO2
vs.
SO2
14
Let’s Try It!
• Are the molecules polar or not?
• A) CH2Cl2
a) A only
b) B only
c) Both
d) neither
15
Molecule Polarity H2O
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.
16
Noncovalent Interactions
Intermolecular Forces
Intermolecular Forces
• Weaker forces than covalent bonds that are
responsible for:
• Physical state of substance
• Boiling and melting points
• Viscosity, solubility, density
• 3-D structure and function of proteins and DNA
Types of Intermolecular Forces
• Ion-Ion Interaction –
▫ Present in all ionic compounds.
▫ Strongest of all types of intermolecular forces.
18
Types of Intermolecular Forces
• Dipole-Dipole
▫ Present in polar molecules
▫ A polar molecule could be represented as a
small dipole ( tiny magnet)
19
Types of Intermolecular Forces
• Ion-Dipole Interaction
▫ Present when ionic compounds are in solution.
20
Types of Intermolecular Forces
• Hydrogen Bond
▫ Present in F, O, and N when they have a hydrogen
directly attached to them due to the very polar bond.
▫ The hydrogen bond is formed between a H attached
to an F, O, or N and the F, O, or N of a neighboring
molecule.
▫ Stronger than dipole-dipole interaction.
21
The Hydrogen Bond
22
Hydrogen Bonding
• The “stickiness" of water molecules, resulting in beads of
water is due to hydrogen bonds.
• They also cause surface tension in water. (insects walking
on water)
• Their order in solid water (ice) results in a lower density, so
ice floats on water; other solid states generally sink in their
liquid states.
• Water being a liquid and boil at 100 C, while similarly sized
methane is a gas.
• Water controls global temperatures.
• Read Hydrogen bond in DNA
24
London Dispersive Forces
aka
Induced Dipole-Induced Dipole Forces
• Important for nonpolar molecules.
• Compare the physical state of the halogens:
▫ F2
Cl2
Br2
I2
• Something has to be present that makes the
molecules feel some form of intermolecular
attraction
London Dispersion Forces
• Attraction is due to instantaneous, temporary
dipoles formed due to electron motions.
dispersion forces increase when
the number of electrons in a molecule increases, and
the surface area of a molecule increases
Larger the molecule, more dominant London dispersive
forces become (stronger).
25
Intermolecular Forces
• Is it ionic?
• yes-IONIC ; in solution? Yes (ion-dipole) ; no
(ion-ion
• No-COVALENT; polar? Yes (dipole-dipole);
• No; London dispersion
• H-bonding?
26
27
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