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Thread: Hydrogen bonding

  1. #1 Hydrogen bonding 
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    The issue of hydrogen bonding has me a bit flummoxed.

    My understanding of bond polarity is that the combination of forces acting on bonding electrons, exerted by the two nuclei, pulled bonding electrons closer to one or other of the 2 bonding atoms.

    Im having to explain the unusual properties of water put down to hydrogen bonding.

    However when I looked at the properties of O and H atoms I was at a loss to explain why the OH bond should be so polar with the O being negative.

    Consider that electronegativity will be effected by electron shielding and distance of bonding electrons to the nucleus. Oxygen is subject to greater loss of pulling power on both these counts. Admittedly there are more protons in the oxygen nucleus when compared with the H nucleus, suggesting that there ought to be more pull there.

    But when I looked at the 1st ionisation energies of the 2 atoms, a suggestion of how keen an atom is to hold onto its electrons, I found that H and O have an almost identical value in KJ/Mol. I would expect a polar bond to involve two atoms with wildly different values.

    Have I gotten something elementary wrong, am i underestimating the importance of the difference in the ratio of protons to electrons in the two atoms or is it something else having an effect that i haven’t found.

    Help appreciated from teh gurus.


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  3. #2 Re: Hydrogen bonding 
    Moderator Moderator AlexP's Avatar
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    Quote Originally Posted by Arnan
    But when I looked at the 1st ionisation energies of the 2 atoms
    I could be wrong here, but I really think that once bonding occurs, electronegativity difference is much more significant than ionization energies. The EN of oxygen is 3.44 and the EN of hydrogen is 2.20, for a difference of 1.24. That difference makes the bond pretty polar, thus making hydrogen bonding possible.


    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  4. #3  
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    I support Chemboy's statement that an element's electronegativity is the key factor here, not its ionization energy. Ionization energy is a property of a sole atom, one that is not bonded to any other. This is not the case for an atom that is part of a molecule.

    Now consider the carbon-hydrogen bond. It is considered non-polar. Shared electrons are hold equally between the two atoms.

    In the oxygen-hydrogen bond, the oxygen atom has a similar electron configuration with carbon. Both have filled their bonding shells. But oxygen has the two extra protons in its nucleus. That is what makes it the more electronegative element, and therefore the oxygen-hydrogen bond is quite polar.
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  5. #4  
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    You might look at it like this: instead of considering the 1st ionization energy, consider the "deionization" energy of pulling an electron off on an O- vs. an H-. It takes a lot more energy to pull the extra electron off the oxygen anion than the hydride.
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  6. #5  
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    upon reading about electron affinity and the pauling scale things make more sense.

    Thanks gents.
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