When Was The Atomic Theory Formulated?

This was during WWII by Albert Einstein discovering how to split the atom. He was devastated at his discovery knowing that it could be turned into a weapon of mass destruction. You can tell we had old, very old textbooks, as my father only married after the war, and our science books still made the claim the atom could not be split.  The net sure has changed the way information is released. And for some reason, what once made the national news is now but a mere mention