Look at these pictures of Hiroshima and Nagasaki before and after the atomic bomb was dropped and think about the power of these weapons and the people who decided to use them.
If you get done blowing up cities, try this link but do the stuff below once you get there. It's a good review.
Choose 40 questions
Select protons, neutrons and nucleons
Choose 'yes' for rounding the element's atomic weight and get started.