Why can't Chlorine and Iodine form an ionic bond?

Well, they are in the same row of the periodic table (row 7) and both have a -1 charge, so they both have a spare electron that they want to donate. So they are both trying to give it away, they aren't going to take on another one from someone else! Whereas someone like hydrogen has a +1 charge, so they want one electron to get to noble gas configuration, so that is why HCl and HI work so well together. One has an electron it wants to get rid of to be neutral, and the other wants to have an electron to be neutral, so they buddy up.