Shapes and Polarity
Vocabulary
• VSEPR model
• Linear
• Trigonal planar
• Tetrahedral
• Trigonal pyramidal
• Bent
• Trigonal bipyramidal
• Octahedral
• Polarity
• Non-polar covalent
bond
•
•
•
•
•
•
•
•
•
Polar covalent bond
Ionic bond
Symmetrical
Asymmetrical
Intermolecular
Interaction
Van der Waals Force
London Dispersion
Force
Dipole-Dipole
Hydrogen Bonding
Objectives
1. The VSEPR model is used to determine
molecular shape.
2. Describe how electronegativity is used to
determine bond type.
3. Compare polar and nonpolar covalent bonds
and polar/nonpolar molecules.
4. Define the interactions between molecules.
5. Draw and apply the intermolecular forces
between molecules.
VSEPR Model
• The shape of a molecule determines many
of its physical and chemical properties.
• Molecular geometry (shape) can be
determined with the Valence Shell Electron
Pair Repulsion model, or VSEPR model
which minimizes the repulsion of shared and
unshared atoms around the central atom.
VSEPR Model (cont.)
• Electron pairs repel each other and
cause molecules to be in fixed
positions relative to each other.
• Unshared electron pairs also
determine the shape of a molecule.
• Electron pairs are located in a
molecule as far apart as they can be.
VSEPR Model (cont.)
VSEPR Model (cont.)
VSEPR Model (cont.)
Section 8.4 Assessment
The two lone pairs of electrons on a water
molecule do what to the bond angle between
the hydrogen atoms and the oxygen atom?
A. They attract the hydrogen atoms and
increase the angle greater than 109.5°.
A
0%
0%
A
B
C
D
0%
0%
D
D. They create resonance structures
with more than one correct angle.
C
C. They do no affect the bond angle.
A.
B.
C.
D.
B
B. They occupy more space and squeeze
the hydrogen atoms closer together.
Section 8.4 Assessment
The sp3 hybrid orbital in CH4 has what
shape?
A. linear
B. trigonal planar
D
A
0%
C
D. octahedral
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. tetrahedral
Electronegativity and Bond Character
Polarity: Having poles (Having opposite ends)
e.g. magnets have a North and South pole
There are 2 ways to share electrons but they can also be
transferred
-“non-polar” covalent bonds: equal sharing of the e- pair
-polar covalent bonds: unequal sharing of the e- pair
- ionic bonds: transfer of e-’s from a metal to a nonmetal
Electronegativity and Bond Character
• Noble gases are not listed with electronegativity
because they generally do not form
compounds.
Electronegativity and Bond Character
Bond type is based on the electronegativity difference
between the two bonded atoms.
0 to 0.4 = nonpolar covalent bond
0.5 to 2.0 = polar covalent bond
Above 2.0 = ionic bond
Practice Problems: Determine the type of bond that
forms between the atoms in the following compounds.
CO2
NaCl
CH4
Electronegativity and Bond Character
• Unequal sharing of electrons results in a
polar covalent bond.
• Polar covalent bonds form when atoms pull
on electrons in a molecule unequally.
• Electrons spend more time around one atom
than another resulting in partial charges at
the ends of the bond called a dipole.
Polar Covalent Bonds (cont.)
• Covalently bonded molecules are either
polar or non-polar.
• Non-polar molecules are not attracted by an
electric field.
• Polar molecules align with an electric field.
Polarity of Molecules
A polar molecule is like a magnet; one side is slightly
positive and the other side is slightly negative.
(Polar molecules are also known to have dipole moments.)
Polarity depends on the shape and symmetry of the
molecule. -symmetrical molecules = nonpolar
- asymmetrical molecules = polar
Polar molecules are moved by static charges.
Polar Covalent Bonds (cont.)
• Compare water and CCl4.
• Both bonds are polar, but only water is a
polar molecule because of the shape of the
molecule.
Properties of Covalent Molecules
Covalent Molecules:
- Insulators of electricity
- formed between two nonmetal
- usually have low melting points
- solubility in water varies: (polar =dissolve; nonpolar =
insoluble)
-forms covalent cystalline solids.
For a compound to to conduct electricity it has:
1. Charged Particles (ion)
2. Particles Free to Move (liquid or aqueous phase)
The Strength of Covalent Bonds
• The strength depends on the distance
between the two nuclei, or bond length.
• As length increases, strength decreases.
The Strength of Covalent Bonds (cont.)
• The amount of energy required to break a
bond is called the bond dissociation
energy.
• The shorter the bond length, the greater the
energy required to break it.
Properties of Ionic Compounds
Ionic compouds…
-conduct of electricity when dissolved water or are
melted.
-formed between metal and nonmetals
-have high melting points
-usually very soluble in water
-form ionic crystalline solids (lattice)
INTERmolecular Attractions
In order for molecules to stick together, they must be
attracted to one another. The weak attractions between
one molecule and another are called Van der Waals
forces.
They cause gas particles to stick together and condense
at low temperatures.
London Dispersion Forces (LDF’s)
There are three types of intermolecular forces:
1.
London Dispersion forces:
– caused by random electron motion
– generally stronger with more electrons in the
molecule
− exist between all types of molecules
− This force causes Br2 to be a liquid and I2 to
be a solid at room temperature.
Dipole Interaction Forces
3. Dipole-Dipole interactions:
– caused by the attraction of the positive side
of one polar molecule and the negative side of
a different polar molecule
Hydrogen Bonds
Hydrogen Bonding is a special type of dipole
interaction.
This attraction between molecules is not a
chemical bond. Electrons are not being shared or
transferred but the interaction is SUPER
STRONG (strong forces between molecules)
Happens between H and N, O or F
Intermolecular Forces
Substances that contain stong intermolecular forces
have a higher melting point and a higher boiling point.
Liquids containing strong intermolecular forces have
higher surface tension and a higher viscosity.
Surface Tension
Section 8.5 Assessment
The force between water molecules is
what kind of intermolecular force?
A. induced dipole
B. hydrogen bond
D
A
0%
C
D. partial dipole
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. sigma bond
Section 8.5 Assessment
What kind of bond occurs within a
molecule with unequal sharing of electron
pairs?
A. ionic bond
A
0%
D
D. polar covalent bond
C
C. non-polar covalent bond
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. sigma bond