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Published byDennis Chandler Modified over 8 years ago
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Unit 6: Covalent Bonding Intermolecular Forces
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Intra- versus Inter- molecular Forces Intra (means “within”) and refers to the forces that hold atoms together in a molecule. –Covalent –Ionic –Metallic Inter (means “between”) and refers to the forces that hold billions of molecules together –A glass of H 2 O
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Definition of IMF Attractive forces between molecules. Much weaker than chemical bonds within molecules.
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Intermolecular Forces (IMF) The connections that are broken when a solid melts to a liquid and a liquid turns into a gas. They are much weaker than intramolecular bonds. There are 3 main types of IMF’s and all depend on the polarity of the molecule. The more polar a molecule is the “stickier” it is, and thus it is more attracted to the molecules near it.
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Dipole Dipole Forces Force that holds polar molecules together. The positive end of a molecule is attracted to the negative end of another molecule. These forces have ~10% the strength of an ionic bond. +-+-+-+-+-
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Dipole Interactions
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Hydrogen Bonding A special type of dipole dipole force that occurs between Hydrogen in one molecule and a Nitrogen, Oxygen or Fluorine atom in another molecule. H-bonding is the strongest type of intermolecular force and gives water an extremely high boiling point.
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Van Der Waals (Dispersion) Forces These forces hold non-polar molecules together. At any given moment electrons distribution within a molecule can be uneven causing a momentary dipole. -- ++ Creates temporary dipole
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London Dispersion Forces/Van Der Waals Present in all atoms and molecules Major type of IMF for non-polar molecules View animation online.animation
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Van Der Waals (Dispersion) Forces They are the weakest forces These forces account for the lower boiling and melting points for non-polar covalent molecules because they are so weak. Molecular weight is also a factor for non- polar molecules Low MW – less dispersion Gas! Higher MW – more dispersion Liquid!
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Determining IMF – Depends on Polarity NCl 3 –polar = dipole-dipole = dipole-dipole CH 4 –Nonpolar = dispersion HF Polar – with hydrogen attached to small electronegative atom = hydrogen bonding
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IMF tutorial link IMF tutorial link
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