I screwed up on a true/false question that said lithium is the most active alkali metal. I said false, because I remembered that the lower you go in this group, the more explosive the element is. It seems, however, I was thinking of reactivity and not activity. I know about the activity series, and it's something I wouldn't like to memorize, but I would a deeper explanation on the difference between activity and reactivity. Why do we have these two terms?