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Why are Molecular Substances in all States at STP?
Because of:
Intermolecular Attractive Forces (aka “van der Waals”)
• They are what make solid and liquid molecular y
q
compounds possible.
• They are the weakest among all attraction forces (Intra‐molecular and intermolecular) and are called van der Waal’s forces ‐ there are two kinds: • Nonpolar
And ….
• Polar
Nonpolar IMAF’s
London Dispersion Forces (LDF) • weakest of all, • caused by motion of e‐
• come from the temporary attractions between come from the temporary attractions between
induced dipoles in a non‐polar molecule. These are polar regions of the molecules that are formed as the electrons are moving around the molecule.
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London Dispersion Forces
London Dispersion Forces
While the electrons in the 1s orbital of helium While the electrons in the 1s
orbital of helium
would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom.
At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.
London Dispersion Forces
London Dispersion Forces
Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.
London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.
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London Dispersion Forces
• These forces are present in all molecules, whether they are polar or nonpolar.
• The tendency of an electron cloud to distort in this way is called polarizability.
Factors Affecting London Forces
Factors Affecting London Forces
• The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n‐
p
pentane tend to have stronger g
dispersion forces than short, fat ones (like neopentane).
• This is due to the increased surface area in n‐pentane.
Nonpolar IMAF’s
 The strength of dispersion forces tends to increase with increased molecular weight. The bigger the nonpolar molecule, the stronger the LDF’s are Larger atoms have larger electron clouds, which are easier to polarize.
See?
Small non‐polar molecules are gasses at room temperature (ex Methane: CH4).
temperature (ex. Methane: CH
)
Medium‐sized non‐polar molecules are liquids at room temperature (ex. Octane: C8H18).
Big non‐polar molecules are solid at room temperature (ex. Paradichlorobenzene: C6H4Cl2).
 The closer the molecules are, the stronger the LDF’s are.
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#2. Dipole interactions
#2. Dipole Interactions
• Occurs when polar molecules are attracted to each other.
– positive region of one molecule attracts the negative region of another molecule.
g y
g
p
• Slightly stronger than dispersion forces.
• Opposites attract, but not completely hooked like in ionic solids.
 
H F
 
H F
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#3. Hydrogen bonding
Which Have a Greater Effect?
Dipole‐Dipole Interactions or Dispersion Forces?
• If two molecules are of comparable size and shape, dipole‐dipole interactions will likely be the dominating force.
• If one molecule is much larger than another, dispersion forces will likely determine its physical properties.
Hydrogen Bonding
– a) covalently bonded to a highly electronegative
atom such as F, O, and N. This is a very strong dipole.
– b) is also weakly bonded to an unshared electron pair of a highly electronegative atom in a molecule next to it.
• This is the strongest of the intermolecular forces.
Hydrogen bonding allows H2O to be a liquid at room conditions.
(Shown in water)
+ H O
H +
• …is the attractive force caused when a hydrogen atom is: H O
H
This hydrogen is bonded covalently
to: 1) the highly negative oxygen,
and 2) a nearby unshared pair.
• How do Van der Waals forces change as molecular size (mass)
IMAF’s in Water
increases ? As Molecular Size ______________ the strength
of Van der Waals Forces _____________.
• How do Van der Waals forces change as molecules are
brought closer together ? As distance between molecules
____________
VdWF _________________
• How do Van der Waals forces affect melting (freezing) points,
boiling (condensation) points, and vapor pressure?
As VdWF get stronger, melting (freezing) points and boiling
(condensation) points will ____________ and vapor pressure
will _______________.
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Intermolecular Forces Affect Many Physical Properties
Example Questions:
1. Which has the strongest Van der Waals Forces
(Intermolecular Attractions)?
1) He (l)
2) Ne (l)
3) Ar (l)
4) Kr (l)
The strength of the attractions between particles can greatly
particles can greatly affect the properties of a substance or solution.
2. Which has the highest melting point/boiling
point/lowest vapor pressure ?
1) He
2) Ne
3) Ar
4) Kr
3. Which has the lowest melting point?
1) CH4
2) C2H6
3) C3H8
4) C4H10
How Do We Explain This?
Molecule‐Ion Attractions – In Solution
10_210
100
H2 O
Group 6A
Boiling point ( °C)
HF
0
H2Te
SbH3
Group 7A
H2Se
NH 3
H2S
HCl
Group 5A
Group 4A
SnH4
HBr
GeH4
PH3
– 100
HI
AsH3
SiH4
CH4
– 200
2
3
4
5
• The nonpolar series (SnH4 to CH4) follow the expected trend.
• The polar series follows the trend
follows the trend from H2Te through H2S, but water is quite an anomaly.
Molecule –
Ion
Attractions
Period
The boiling points of the covalent
hydrides of the elements in Groups
4A, 5A, 6A, & 7A.
Order of Intermolecular attraction strengths
Ionic
Bonds
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A Handy Chart For You
1) Dispersion forces are the weakest
2) A little stronger are the dipole interactions
3)) The strongest is the hydrogen bonding
h
i h h d
b di
4) All of these are weaker than ionic bonds
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Another Handy Chart
Can You Do These Things?
1. Identify whether a compound is molecular, ionic, metallic or a network solid based on its properties.
2. Draw dot diagrams of simple molecules.
3. Draw structural formulas of simple molecules.
4. Determine the shape of simple molecules.
h h
f
l
l l
5. Determine if simple molecules are polar or nonpolar.
6. Draw the dipole moment of polar molecules and ID charged ends.
7. Determine the type of IMAF that attracts specific simple molecules to each other.
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