Download Intermolecular Attractions

Intermolecular Attractions
BONDING AND VSEPR THEORY
STRUCTURES OF SOLIDS AND LIQUIDS
Electron Dot (Lewis) Diagrams
Explain Chemical Bonding
CHEMICAL BONDS – OCCUR WHEN
ELECTRONS ARE TRANSFERRED OR SHARED
BY ELEMENTS SO THAT THEY EACH
BECOME MORE STABLE
17%
17%
17%
17%
18
17%
8
1. 0
17%
6
How many electrons do most atoms want in their
outer energy level to be stable?
2. 1
3. 2
4. 6
5. 8
2
1
0
6. 18
Bonds that form between two nonmetals are
which type?
1. Ionic
2. Covalent
33%
33%
33%
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3. Metallic
Drawing Electron Dot Diagrams
 Electrons usually stay in pairs when bonded.
Bonding pairs – pair of electrons that form the bond
- can be represented as a line segment
Lone (or unbonded) pairs – pairs of electrons that are
not involved in bonds and are shown as dots
How many bonding pairs are in the following
compound?
1. 1
14%
14%
14%
14%
14%
14%
14%
2. 2
3. 3
4. 4
5. 6
6. 9
18
9
6
4
3
2
1
7. 18
How many lone pairs are in the following
compound?
1. 1
14%
14%
14%
14%
14%
14%
14%
2. 2
3. 3
4. 4
5. 6
6. 9
18
9
6
4
3
2
1
7. 18
How many bonding pairs and lone pairs are in
the following compound?
1. 6 bonding pairs,
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6b
18 lone pairs
2. 12 bonding pairs,
18 lone pairs
3. 12 bonding pairs,
36 lone pairs
4. 6bonding pairs, 6
lone pairs
Drawing Electron Dot Formulas for Compounds
 Exceptions:
Hydrogen only needs
2 electrons (1 bond)
Boron tends to need only
6 electrons (3 bonds)
Single atoms go in the center
If more than one single atom,
middle atom central atom
Draw the electron dot formula. Then state how
many bonding and unbonding pairs are present.
 A) NBr3
 B) Water
 C) Chlorite ion (ClO2- )
 D) CF2Cl2
Multiple Bonds
 If there are not enough electrons to form full octets,
multiple bonds may need to be formed.
Draw the electron dot formula
 E) O2
 F) CO2
Resonance Structures
 If there are more than one possibility, resonance
structures are drawn.
 Resonance structures show possible locations of
the bonds. In reality the electrons exist as an
average of the two structures – splitting time equally
between them.
Resonance Example
 Each resonance structure is shown followed by the
combination with the double bonds shown with a
dotted line as one of the bonds.
Draw the electron dot formulas including
resonance structures
 G) SO2
 H) N2O
What is the name of the property that describes
the tendency of an atom to attract electrons when
bonded to another atom?
20%
20% 20%
20% 20%
1. Ionization energy
2. Conductivity
3. Electronegativity
4. Metallic Character
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5. Bond length
Classifying Bond Types
 Chemical bonds can be classified by how much the
bonded electrons are shared or are not shared by the
elements involved.
• Electronegativity: The ability of one atoms in a
molecule to attract electrons to itself.
• Wolfgang Pauling set electronegativities on a scale
from 0.7 (Cs) to 4.0 (F).
• Electronegativity increases
•
•
across a period and
down a group.
Electronegativities of Elements
Electronegativity
Bond Classification based on Electronegativity
Difference
 As the difference in electronegativity increases,
electrons are less equally shared and become more
polar.
Bond Classification based on Electronegativity
Difference
Type of Bonds
Electronegativity
Difference
 Nonpolar covalent
 x ≤ 0.4
 Polar covalent
 0.4 < x < 1.8
 Ionic
 x ≥ 1.8
Classify the bond between the following
elements: Cl and Cs
1. Ionic
2. Polar Covalent
3. Nonpolar Covalent
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Classify the bond between the following
elements: C and H
1. Ionic
2. Polar Covalent
3. Nonpolar Covalent
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0%
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0%
Classify the bond between the following
elements: N and O
1. Ionic
2. Polar Covalent
3. Nonpolar Covalent
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Interactions between Molecules
INTERMOLECULAR FORCES
Polarity of a Compound
 Like bonds, compounds themselves can also be
classified as polar or nonpolar.
 Polarity is based on:
 Difference in electronegativity of atoms within a compound
 Symmetry of the compound
Nonpolar Compounds
- Diatomic molecules are always nonpolar. (ex. F2)
- Also, compounds that are totally symmetric may be
nonpolar as well. (ex. CCl4)
Nonpolar Compound – the bonds are polar but
the dipoles cancel out since the compound is
symmetrical (tetrahedral)
Nonpolar Compound – the bonds are polar but
the dipoles cancel out since the compound is
symmetrical (linear)
Polar Compounds
 Polar compounds have one side of the compound
that is more positive and another side that is more
negative.
Polar Compounds
Polar Compounds
Polar Compounds
BF3 = Polar or Nonpolar
1. Polar
2. Nonpolar
0%
1
0%
2
CH3F = Polar or Nonpolar?
1. Polar
2. Nonpolar
0%
1
0%
2
CF4 = Polar or Nonpolar?
1. Polar
2. Nonpolar
0%
1
0%
2
Br2 = Polar or Nonpolar
1. Polar
2. Nonpolar
0%
1
0%
2
PBr3 = Polar or Nonpolar
1. Polar
2. Nonpolar
0%
1
0%
2
Intermolecular Forces
 Intermolecular Forces are forces that exist between
two molecules that hold them together.
 Intermolecular Forces are caused by charge
differences and polarity (because positive and
negatives attract)
 The stronger the polarity, the stronger the attraction
between molecules.
Intermolecular Forces
 The stronger the polarity, the stronger the attraction
between molecules.
 The strength of the attraction between molecules
determines properties such as:





Boiling point
Melting point
Surface tension
Cohesion
Capillary action
Types of Intermolecular Forces
 Three major types of intermolecular forces:
 Dipole-Dipole Interactions

Hydrogen Bonds

Dispersion Forces
Dipole-Dipole Interaction
 Occurs in polar molecules.
 Positive pole of one molecule is attracted to the
negative pole of the next molecule.
Hydrogen Bonds
 Occurs in polar molecules when the hydrogen atom
is attracted to the more electronegative nitrogen,
oxygen, or fluorine atom of another molecule.
Dispersion Forces
 Dispersion forces are the weakest type of
intermolecular forces because they exist between
nonpolar molecules.
 Usually, the electrons are shared equally. But
because electrons are constantly moving, sometimes
a temporary dipole forms when all the electrons
are on one side of the molecule.
 This temporary dipole would cause an attraction
with another temporary dipole.
Summary of Intermolecular Forces (from
strongest to weakest)
What kind of intermolecular force would exist in
H2O?
1. Hydrogen bonding
2. Dipole-Dipole
3. Dispersion
0%
1
0%
2
0%
3
What kind of intermolecular force would exist in
PCl3?
1. Hydrogen bonding
2. Dipole-Dipole
3. Dispersion
0%
1
0%
2
0%
3
What kind of intermolecular force would exist in
Br2?
1. Hydrogen bonding
2. Dipole-Dipole
3. Dispersion
0%
1
0%
2
0%
3
What kind of intermolecular force would exist in
NH3?
1. Hydrogen bonding
2. Dipole-Dipole
3. Dispersion
0%
1
0%
2
0%
3
Melting and Boiling Points
 In order to melt or boil a substance, intermolecular
forces must be broken.
 Therefore, weaker intermolecular forces require
less energy and have lower melting and boiling
points.
Therefore, stronger intermolecular forces require
more energy and have higher melting and boiling
points.
Which type of intermolecular force would have
the lowest boiling point and exist as a gas at room
temperature?
1. Ionic Bonds
2. Dispersion Forces
3. Hydrogen Bonds
4. Covalent Bonds
5. Dipole-Dipole
attractions
0%
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0%
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2
3
0%
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4
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Which type of intermolecular force would have
the highest melting point and always exist as a
solid at room temperature?
1. Ionic Bonds
2. Dispersion Forces
3. Hydrogen Bonds
4. Covalent Bonds
5. Dipole-Dipole
attractions
0%
1
0%
0%
2
3
0%
0%
4
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Which compound would have the lowest boiling
point and exist as a gas at room temperature?
1. H2O
2. N2
3. CuI2
4. CO2
5. LiF
0%
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0%
0%
2
3
0%
0%
4
5
Which compound would have the highest melting
point?
1. NH3
2. NaBr
3. F2
4. CO2
5. NH3
0%
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0%
0%
2
3
0%
0%
4
5
Effects of Molecular Mass on Melting and Boiling
Points
 The higher molecular masses will have higher
melting and boiling points because they have more
electrons that form temporary dipoles.
 Acts as the tiebreaker if the forces are the same types
– highest mass has the strongest force (highest
melting and boiling point).