[miaou5]

According to my notes, acidity of a bond increases with increasing S character--in other words, bonds with more S character are more likely to be able to give up a proton. But according to my professor, atoms that are closer together form stronger bonds, so aren't bonds with increasing S character also stronger bonds, since they're the shortest? If they're so strong, why do they give up a proton more easily?

[Corribus]

This question was answered here before (many many years ago!).

http://www.chemicalforums.com/index.php?topic=4261.0

Also, not to add unnecessary confusion, but your professor is wrong when (s)he says that when atoms are closer together, they form stronger bonds.  This is usually true, but not always.  Fluorine, F₂, actually forms a weaker sigma bond than Cl₂ - fluorine is so small that when it forms bonds, all of its lone pairs have repulsive interactions that destabilize the bonds.  (Bond enthalpy for the F₂ bond is 158 kJ/mol and for Cl₂ it is ~240 kJ/mol. The lengths of these bonds are 0.142 nm and 0.199 nm, respectively.  So you see, fluorine has a shorter bond but it is also weaker.) 

[miaou5]

That makes things so much clearer--thank you for the info and the link.  I really appreciate it!