[micro_star]

good morning ..

hi every body >.

i have a quastion about the acidity of bond ...

which of the following is more acidic >>>?
  C-F  or  C-I
and why .?

please help me ...

it is very important to me to know th answer

[nj_bartel]

Acidity tends to increase with conjugate base stability (typically increased electronegativity of the conjugate base when the conjugate base is a monoatomic ions).  This increase in stability with increase in electronegativity is due to the greater ability of the conjugate base to balance the negative charge.

Acidity also increases (when your conjugate base is monoatomic) with increased atomic radius of the conjugate base.  This decreases the bonding strength between the conjugate base and the proton.  This second effect is the dominating effect when you go downward within a group.

[spirochete]

Although it might be possible to guess what you mean by that you really should clarify your question.  Acidity refers to the tendency of an atom to lose a proton.  There are no protons in the partial molecules you're implying. 

[nj_bartel]

Oops, read that as HF and HI.

[sjb]

Although it might be possible to guess what you mean by that you really should clarify your question.  Acidity refers to the tendency of an atom to lose a proton.  There are no protons in the partial molecules you're implying. 

Not always. Lowry-Broensted acid theory points towards what you're saying, but there is also things like Lewis acid/bases - the tendency to accept or donate electron pairs respectively, so in that case as C-F is more polar, and sucks electrons away I'd suggest C-F is more acidic..?