Some one asked me this (above)question and thats what i had to say about it;

 

HF is a molecule in which F is more electronegative than H, that is, F has a higher tendency to attract electron than H, so the cosequence is that this molecule has a dipole, negative over F and positive over H.

 

B:--> H--------F --------> BH+ + F- (I hope you can imagine the arrows and dipoles over HF molecule)

 

A nucleophile attach the H dipole and the bond breaks heterolytically so that both electron in the bond go to the same atom in this case to flourine.

 

I was going fine until here and got stuck as to how to explain further. I will appreciate any comments from you users. I get the feeling i must mention the conjugate base!?

the simplest answer is probably that the H-F bond is closer to being ionic than the H-H bond. It's not so much the electronegativities but the difference in electronegativities which makes the difference. In H-H there is no difference so the bond can be said to be entirely covalent. Something like H-Cl is almost ionic because of the uneven sharing of the electrons (HCl is ionic in water but HCl as a gas contains very polar covalent bonds). H-F is a weaker acid than H-Cl, because the H-F bond is stronger than the H-Cl bond, mostly because the H and Cl atoms are very different in size. The extreme example (although not an acid) is NaCl, which is totally ionic, and the electrons are not shared at all but entirely transferred to the chloride ion.

One has to consider the multiple definitions of acid in this case, but since we are discussing a hydrogen halide anyway, any will be appropriate. The fluorine, being highly electronegative in comparison to the hydrogen, pries the electrons away from the hydrogen. This is now an electron deficient hydrogen, which satisfies the lewis acid definition (which is the most useful definition once you get into organic chemistry). For the more well known arrhenius acid definition, this electron deficient hydrogen is fairly happy to break its strained bond to fluorine and attach itself the the lone pairs of electrons on the oxygen of water, forming a hydronium ion.