We know it reacts easily (I'm not sure if that's the correct word, I'm not very good in chemistry, especially in English). Kelsier says that you cannot keep it for too long after swallowing, because acid in your stomach reacts with it. And it isn't a very strong acid...
Most metals easily reacting with acid are rather light (weight, not colour), as far as I know. But atium is special, so it can be different.