I in case of CF3I has a partial charge of +1

and in case of CH3I has a partial charge of -1.

Why?

I know it is related to electronegativity but please explain!

IN case of CF3I it dissociates as CF3- and I+ the charge on iodine is +1 whereas CH3I dissociates as CH3+ and I- the charge in case of I- is -1.see as the electronegativity of flourine is the highest therefore CF3- is formed and in case of CH3I hydrogen electronegativity is less than iodine therefore it forms as CH3+ and I-