Who discovered nuclear weapons?

Nuclear weapons were developed by scientists working in the U.S. during World War 2. We were the ones who first used them. A number of scientists around the world realized that a huge amount of energy could be developed very quickly by nuclear means, and this meant a weapon. This was known in the 30's, and almost certainly before that. The atomic bomb wasn't really "invented" by anyone in particular. It was the team put together by the U.S. that first put a bomb together and tested it. Then we built a pair of them and dropped them on Japan.