As the period of the element increases in the group, the radius of the atom increases. The valence electrons are farther away from the nucleus. So the electropositivity increases and electronegativity decreases.
So electronegativity decreases in this order: F, Cl , Br, Iodine.
HF is more ionic(polar) and HI is more covalent. Greater the difference between the electronegativities of the two atoms in the bond, more polar the bond is.