I read that in alkali metal halides, the formation enthalpy for fluorides become less negative as we go down the group, whereas the reverse is true for chlorides, bromides and iodides.
Why is it so? In my opinion, the formation enthalpy should increase with the stability which according to me, will increase as the cation's size increases and anion's decreases.
Therefore the trend is understandable in chlorides and others, but what about fluorides? May be it is due to its extremely small size but I can't find a suitable reason.