Why HF is a strong acid in period but HI is a strong acid in group?

Strength of acid

 

In a period left to right electronegativity increases, F is most electronegative (4.0) in a same period. So HF is a strong acid, it can easily release proton. In a group top to bottom, we will consider dissociation energy. As we move from top to bottom in a group dissociation energy decreases, hence HI has least dissociation energy as a result it becomes stronger  acid as compared to HF.

 

 Period:    CH₄, NH₃, H₂O, HF                                           Group

                                                                                             HF

                                                                                             HCl

                                                                                             HBr

                                                                                              HI