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HF is a weak acid why? because when looking at an acid we always look at the fact that the acid dissociates to lose a proton.
In the following pair below we have:
Conjugate acid base pairs differ based on the presence of a proton.
F- is our conjugate base, while HF is the acid.
Whenever an acid loses a proton we get the conjugate base
Whenever a base gains a proton we get a conjugate acid.
If you look at the \[\Delta H \] the bond energy you would find that it's really hard to break the F-H bond, why?
fluorine is so much more electronegative than Hydrogen that breaking a bond and placing one electron in hydrogen and one in fluorine would be unfavorable. in fact the result is more likely to be that we get F- as a result and H+ but it's harder to do that b/c that's what Bond dissociation energy is a homolytic cleavage; we have a heterolytic cleavage going on.
also acidity has to do with how stable your conjugate base is. One of these things has to do with the size of your atom. the larger your atom is (larger atomic radius) the more polarizable it will be, it will be easy for it to stabilize the negative charge. and as you go down CL, BR, i, the atoms get bigger, and that means that it's easier for each of them to give up a proton which means that acidity goes up.
another way you could explain this is possibly by Zeffecvite nuclear charge. what does this mean? like when you go across a period we know that the number of protons increases and the # shells stays the same right? but what does this mean? it means that as you go across the period the nuclear charge becomes stronger and the electrons are held more tightly. I believe that this explains the increase in electronegativity, and increasing ionization energy. what this does is that it the atoms become smaller.