How many atoms you need to break to make an atomic bomb?
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Every atom of uranium you split releases about 200 MeV, which is a tiny unit of energy for a human being, but a lot for an atom. (It’s about as much energy as you’d need to visibly move a speck of dust, which doesn’t seem like much, but atoms are much, much smaller than specks of dust. In terms of scale it’s like an ant being able to make a Space Shuttle wobble by kicking it.) To make an explosion, you need to make lots and lots of atoms split.
You can use Google to tell you how many atoms you’d need to split to get a given amount of explosive output. For example, for the Hiroshima bomb, which was 15 kilotons of TNT, that is [~2 x 10^24 atoms](https://www.google.com/search?q=15+kilotons+of+TNT+%2F+200+MeV+%3D+%3F&oq=15+kilotons+of+TNT+%2F+200+MeV+%3D+%3F). If you want to know how much that might weigh, just multiply that number (2e24) by the weight of the atom you are splitting — [2e24 * 235 amu = ~0.8 kg](https://www.google.com/search?q=2e24+*+235+amu+%3D+%3F+kg&oq=2e24+*+235+amu+%3D+%3F+kg). (I think it is pretty neat that Google lets you convert the units this easily.)
So I usually just say “on the order of a trillion trillion” which gives a sense of the immensity of it, even for a relatively small (by modern standards) atomic bomb.
So it’s a lot of atoms. You can think of an atomic bomb as a very specialized engineering device designed to cause a trillion trillion atoms or more to split in less than a millisecond.
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