|MadSci Network: Chemistry|
Mike, 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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