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Homework Help: 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?

  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


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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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