Intermolecular Forces
What Holds A Liquid Or Solid
Together?
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In ionic compounds, all atoms are
equally bonded together in the crystal
What about covalent?
Two Types of Covalent
Compounds

Covalent Network Crystal
- all atoms covalently bonded together equally
- very strong and hard
- ex: diamond, graphite

Covalent Molecular Substance
- individual molecules attracted to one another
through intramolecular forces
- not anywhere near as strong as networks
- ex: water
Intermolecular Forces
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Force of attraction between molecules
in a covalent molecular substance
Three types:
Dipole – Dipole forces
Hydrogen Bond
London Dispersion forces
Dipole- Dipole
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Between polar molecules
Positive pole of one ion attracted to the
negative pole of another
Hydrogen Bonding

Attraction between hydrogen atom in one
molecule and an unshared pair of
electrons on the atom of another molecule
London Dispersion or Van der
Walls Force
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Hold non-polar molecules together
Caused by motion of electrons
Strength of Interaction
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Substances held together by ionic bonds or
covalent network bonds are very rigidly
connected: mostly solid at room temp and
melt at very high temps
Those held by dipole-dipole or hydrogen
bonds are fairly strongly connected: liquid at
room temp or solids that melt at low temps
Those held by london dispersion forces are
weakly connected: gases at room temp; need
to be very cold to become liquid
Ionic vs. Covalent
Ionic
Melting Point
Vaporize
Crystalline
Dissolve in Water
High
No
Yes, hard & brittle
Yes
Covalent Molecular
Low
Maybe
Maybe, soft
If polar covalent
Conduct electricity when
liquid
Yes
No
Conduct electricity when
dissolve in water
Yes
No
Example
Substance A melts at 600oC, dissolves
well in water, and does not conduct
electricity when solid but does when
dissolved in water. Is the substance
held together by ionic or covalent
bonds?
Example

Identify the interparticle forces present
in each substance
a) HI
b) SO2
c) Ne
d) CF4
e) CH3OH

Stronger the intermolecular forces,
- the more solid in nature the substance
- higher the melting and boiling points
- higher the viscosity and surface
tension
- lower the vapor pressure
Metallic Bond

Bond that exists between metal atoms
Electron Sea Model

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Metal atoms give up valence electrons
and form + ions
The released electrons move freely
around the + metal ions
Properties of Metallic Bonds
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Good conductors of electricity – free
electrons
Malleable and ductile – not in rigid
position so ions can be shaped and
drawn into wires
Conduct heat because of free electrons
Lusterous – absorb and emit light in
regular pattern due to free electrons
Alloy

Mixtures of metals
Interstitial Alloy
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One metal in the mixture is a much smaller
atom than the other
Increases the overall density of the metal
Tends to decrease malleability and ductility
Steel is an example: carbon between iron
Substitutional Alloy
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Atoms of similar radius mixed
Density is an average of the pure metal
density
Stays malleable and ductile
Brass is an example: Copper and Zinc