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Intermolecular Forces of Attraction

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Positive of one molecule attracts the negative of another. A Short Visual ... Strongly electronegative elements attract electrons so strongly that the ... – PowerPoint PPT presentation

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Title: Intermolecular Forces of Attraction


1
Intermolecular Forces of Attraction
2
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

3
London Dispersion
  • Weakest of the weak intermolecular forces
  • Caused by random and temporary movement of
    electrons
  • Short-lived, temporary force
  • More electrons greater attractions

4
A Short Visual
  • Three Molecules
  • One molecule has a temporary imbalance in
    electrons
  • Each molecule responds to that imbalance

5
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

6
A Short Visual
  • One end of a molecule is permanently positive and
    one end is permanently negative
  • Opposites attract

d
d
d-
d-
7
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.

8
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9
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

10
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

11
A Short Visual
d-
  • The very polar molecule has a very negative end
    and a very positive end.

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

d
12
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!

13
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.

14
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.
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