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How are atomic radii and H-bond strength related?

  1. Jan 20, 2009 #1
    I answered a question that my professor asked: "Why is the boiling point of HF much higher than that of HCl even though HCl has a larger molar mass?"

    The answer had to do with F being more electronegative than Cl and thus making stronger H-bonds.

    He also said on an aside that it all boils down to atomic radii. What did he mean by that? How are the two correlated?

    Thanks!
    Casey
     
  2. jcsd
  3. Jan 21, 2009 #2
    A good rule to remember is that H-bonds(such as H-O, H-F, and H-N) > dipole-dipole bonds (such as H-Cl in this case). Another way to look at this is by the atomic radii. The radii for H-F is smaller than H-CL, this makes it much harder to break away due to the Hydrogen being so close to the electronegative Fluorine.

    An example for atomic radii, I guess, would be to picture yourself holding someone's hand. Would it be harder to break you two apart while you're closer together or father apart?
     
  4. Jan 21, 2009 #3

    GCT

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    Study the equation for dipole moment - it is somewhat related to the differences in electronegativity as well as the distance between the charges - the greater the dipole moment within a molecule the greater the intermolecular attraction between molecules.
     
  5. Jan 21, 2009 #4
    I will look into that. This is a Materials Science course, so we never really covered this stuff explicitly.

    I will do some Wiki-ing :smile:
     
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