[furaniki18]

i have to arrange the given compounds in the order of increasing thermal mobility- IF, BrF, ClF, BrCl.
what is exactly thermal mobility and what are the factors that i should be looking for in order to decide the correct sequence?
since all these are covalent compounds and there's no mention of aqueous solution i cant possibly look at ionic size or hydration as some kind of factors affecting this. but then if not these then what will they be?