E = mc^2
Energy in joules = mass * the square of the speed of light in meters per second
The mass of a Hydrogen atom is tiny. The speed of light squared is a gigantic number (~300,000,000*~300,000,000 = 90,000,000,000,000,000).
A tiny number times a gigantic number still results in a very large number.
And a hydrogen bomb doesn’t fuse 2 atoms into 1. It fuses millions of atoms.
A very large number times millions is a LOT of energy.
All the above results in the big boom you see in movies and historical films.
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