system
Given the following post, respond as if you're commenting on Reddit:
human
Two questions about nuclear weapons: Why does the reaction cause such a devastating blast? Why does the explosion create the "mushroom" cloud shape?
gpt
When you split an atom, or fuse atoms together (fission/fusion), you are liberating a infinitesimally small amount of energy, which then has to go *somewhere*. Now imagine that little bit of energy is trapped in the middle of an unbelievably large, but shrinking, ball of similar atoms, all of which are feeling the pinch. As individual atoms start to give off their energy, they push outwards with pressure... but the outside of this sphere is also being pushed in with explosives of some sort. This energy from the nuclear reaction on the inside is directed into neighboring atoms, which helps them fission/fuse, releasing more energy. In a tiny fraction of a second, the number of reactions double, then double again, then double again, etc, until the pressure inside balances the pressure outside. Until this moment, the visible explosion hasn't even started yet!

As the atoms release their energy, many extremely energetic photons escape in all directions. The first thing outside of a nuclear bomb is probably a metal shell, so this gets irradiated to the point where it glows hotter than the sun. This, in turn, gives off more radiation, which heats up the air until it glows as well, which in turn... etc, etc, until surrounding the bomb is a gigantic wall of glowing gas, a fluorescing cloud of energetic particles, which have had electrons stripped off and all sorts of other terrible things.

Everything above happened in about 50 nanoseconds.

*Then* the blast begins. All that energy has disperse, and so it's turned into a blast wave, where the air molecules are compressed and travel outwards as a pressure wave. Because the molecules are so hot, they begin to rise, as hot gases are lighter than cool gas. In the process, often a bomb will pick up dirt and debris and rock dust and coat it with radioactive leftovers -- this is "fallout", because eventually the radioactive dust lands back on the ground and screws up.... well, life.

The rising air causes the mushroom shape, just as smaller "explosions" like fireworks and propane tanks going "boom" will do, just in miniature. In addition to the blast wave, the light-energy put out by a nuclear weapon can instantly light things on fire at a tremendous distance. If you watch videos of nuclear testing, entire houses burst into flames from the power of the *light alone* before the blast wave reaches then, tearing them asunder.