Intermolecular Forces of Attraction

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Intermolecular Forces of Attraction
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Intermolecular Forces of Attraction

• Three weak forces
• London Dispersion
• Dipole-Dipole
• Hydrogen Bonding
• One strong force (not technically an IMF, but an
  actual bond!)
• Ionic

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London Dispersion

• Weakest of the weak intermolecular forces

• Caused by random and temporary movement of
  electrons

• Short-lived, temporary force

• More electrons greater attractions

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A Short Visual

• Three Molecules
• One molecule has a temporary imbalance in
  electrons
• Each molecule responds to that imbalance

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Dipole-Dipole

• Still a very weak force, but stronger than London
  Dispersion

• Permanent polar molecule has one positive and one
  negative side

• Positive of one molecule attracts the negative of
  another

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A Short Visual

• One end of a molecule is permanently positive and
  one end is permanently negative
• Opposites attract

d
d
d-
d-
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Another Short Visual

• A quick visual of what this might look like

• Notice there is not a regular pattern of the
  molecules - the attraction is not strong enough
  to hold the molecules in a fixed position.

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Hydrogen Bonding

• A weak force of attraction, but strongest
  compared to the previous two (London dispersion
  and dipole-dipole)
• Occurs with only the most electronegative
  elements N, O, and F.
• Strongly electronegative elements attract
  electrons so strongly that the molecule becomes
  very polar

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Hydrogen Bonding

• The non-bonding pairs of electrons on the N, O,
  or F are strongly attracted to the highly
  positive H on another molecule.

• A quarter bond or half bond is formed between
  the H and the pair of electrons, resulting in a
  much stronger force than the previous forces.

• Requirements (both are necessary!)
• H bonded to N, O, F
• Non-bonded pair of electrons on the N, O, or F

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A Short Visual
d-

• The very polar molecule has a very negative end
  and a very positive end.

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d

• The negative electron pairs are attracted to the
  positive hydrogen.

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Yet Another Visual

• Here is an example of hydrogen bonding at work!
• Notice how the positive hydrogens are attracted
  to the negative electron pairs on the oxygen!

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Ionic

• NOT an intermolecular force of attraction!
• Actual bonds are formed between molecules forming
  a crystal lattice (or network) of atoms.
• Attractive forces between positive and negative
  ions are as strong between molecules as within
  molecules.

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A Short Visual

• Na and Cl- ions are attracted to each other
  because opposite charges attract!
• A network of positive/negative alternating
  charges begins to form.