The problem with this is that if this is the case, then it is either one of the known elements, (although maybe not known on Sel), or it has an atomic number of over 118. And the most noticeable physical property of elements in that row of the periodic table have they aren't very stable stable at all.
Actually, though, this could explain a lot - Atium is held together using Ruin's energy, which allows it to be stable even when by all rights it should be exploding. Incidentally, this would mean that if Ruin's energy was ever removed from Atium (which is what Ruin was presumably trying to do), it would cause the world's most terrifying nuclear explosion, and probably demolish the planet. Which is one more reason Ruin wanted his Atium back.
I still support the "Atium isn't matter" theory (if anybody's read the short story Microcosmic God, I think it's kind of like the impenetrable shield at the end, and if you haven't read the story, you should. It's great.) but I must admit that Atium could work as an element.