MadSci Network: Chemistry

Re: In reference to h-bonds(seecomments)

Date: Mon Jan 26 19:24:37 1998
Posted By: Chris Larson, Post-doc/Fellow Laboratory of Genetics
Area of science: Chemistry
ID: 885329789.Ch


	I am not exactly sure what you are asking here, so I will answer the 
possible question that I think you might be asking.  First, just to
remind you, electronegativity refers to the extent of attraction of an atom
for a shared pair of electrons (shared with another atom in the form of a 
bond) and as such is a measure of the attraction of an atom for electrons
in a covalent bond.  In general, for the representative elements, electro-
negativity increases going across a period (in the periodic table) from 
left to right and going up a group.  Thus since hydrogen is at the top 
right corner of the periodic table, it has one of the higher 
electronegativity values of all of the elements.  And it is also true that
the electronegativity of a given element such as hydrogen varies a little 
depending on the compound in which the element is found.  But I think what 
you are asking is, when hydrogen is bonded to an element more 
electronegative than itself, why is the partial positive charge on hydrogen
more concentrated as the other element gets more electronegative.  If two 
atoms like hydrogen and something else are bonded and the other atom is 
more electronegative the hydrogen, the covalent bond will be polarized and 
the electrons will spend more time near the other element than near the 
hydrogen (this is just the definition of electronegativity).  Thus the 
hydrogen is left with less electrons to neutralize its positive nuclear 
charge and becomes more positive as the elements get more electronegative 
and the electrons spend more and more time near the other element.  Perhaps 
this is what you mean by a more "concentrated" charge.  The other way to 
think about it is that the overal charge on the hydrogen is the sum of the 
positive nuclear charge, which stays the same in sign, magnitude, and in 
its very focused localization on the extremely small and dense nucleus, and 
the negative charge from the electrons, which gets smaller as the electrons 
are pulled onto the other atom.  So as the negative electrons spend less 
time near hydrogen the overall charge gets more positive and the proportion 
that derives from the very small, dense, localized nucleus increases and 
the proportion that derives from the much larger and more diffuse electron 
orbital decreases, so in that sense the charge becomes more "concentrated" 
(it occupies a smaller space).  If that is not what you were asking, please 
feel free to email me directly.

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