[vnaraya3]

In general, I thought that a negative formal charge is stabler on an electronegative element, and a positive formal charge is stabler on a more electropositive element.

Given that nitrogen is the third most electronegative element there is, I am surprised that it so frequently bears a positive charge and rarely bears a negative charge.

For example, why is water so much easier to deprotonate than ammonia (i.e. the pKa of water is much lower than that of ammonia?) I know that oxygen is more electronegative but does this fully explain the difference?

Also, I know that carboxylic acids are acidic because the carboxylate ion is stabilized by resonance - the negative charge is shared between the oxygens. Why aren't amides (i.e. RNH(CO)R') acidic by the same reasoning? I'd understand if they were only slightly less acidic than carboxylic acids (since nitrogen is less electronegative) - but why is it that amides are not even at all acidic?

In general, if nitrogen supposedly is the third most electronegative element, why does it always donate/share electrons (giving it a positive formal charge) but rarely bears a negative formal charge?

[orgopete]

The most simple answer is that first off, electronegativity is wrong. It fails to predict HI is a stronger acid than HF, among other predictions. Secondly, the pKa better represents acidity and their relative positions. In that case ammonia is a very weak acid. As a weaker acid than water, amides should be weaker acids than carboxylic acids.

[Babcock_Hall]

I respectfully disagree that electronegativity is "wrong."  Electronegativity correlates with some physical or chemical properties but not others.  Why should it correlate with acidity?  Another thing to bear in mind is that formal charge is not the same thing as charge.

[vnaraya3]

Thanks for the responses! Assuming then that electronegativity can't explain difference in pKa, what does account for these differences?

[orgopete]

I respectfully disagree that electronegativity is "wrong."  Electronegativity correlates with some physical or chemical properties but not others.  Why should it correlate with acidity?

In that case, I can agree. Electronegativity is not wrong, it just doesn't correlate with acidity.

If you create a table of compounds with bond energy and pKa, the bond energy data corresponds with gas phase reactions and pKa, liquid. They are different. In my model, electron pairs are negative and attractive to protons. However, the nucleus presents a repelling field as well. Both of these forces are functions of charge and inverse square to distance. The result is CH4, NH3, H2O, and HF have an increasing nuclear charge which pulls the electrons closer, the protons follow, and the bond lengths shorten. The combination also increases the nucleus-proton repulsion and the proton-electron pair gap increases. The net effect is as the bond becomes shorter, the acidity increases due to the weaker Coulombic attraction. The same principles are in effect for the haloacids, except the bond lengths are longer because as you move down the periodic table, you are adding an additional shell of electrons.

Create another table with the haloacids, pKa, and standard electrode potential. I predict you will see a good correlation. You might conclude electronegativity corresponds more with the redox reactions than acidity. That is electronegativity correlates more with a change in the number of electrons around an atom than acidity, which does not.

I suggest one should consult pKa tables to predict acidity. It is my opinion this will better predict the atomic effects being questioned.

By the way, if anyone can give me an example showing fluorine as the most electron withdrawing element, please tell me.