answer:1. Uranium emits neutrons, which are very harmful. In the bomb, it reaches critical mass and many times more neutrons are emitted. Standing next to one may not give you radiation poisoning (depending on how long you stand there), but it will dramatically increase your risk of cancer. 2. Many atoms are split. In a normal block of uranium, more neutrons escape to the environment than those that cause further fission events. Once critical mass is reached, few neutrons escape to the environment, so the number of atoms being split rises exponentially, which leads to an explosion. See this resource, which will be able to explain it all for you.