Why does splitting an atom create energy? And why is it so much energy?
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Splitting atoms is a process called fission. During fission, we take a bunch of large atoms that are already radioactive and cram them into a confined space. Radioactive atoms constantly release energy in the form of particles in a process called radioactive decay. If you cram enough of these atoms together in one place, the particles that are being released will sometimes ram into the other atoms nucleus so hard that it causes them to split.
When the nucleus of an atom splits, something interesting happens. You get two new atoms (a collection of neutrons, protons, and electrons) plus some spare neutrons that go flying off to bump into other atoms, possibly splitting them. The interesting thing is that if you add up the weight of the two new atoms (called fission products) and the spare neutrons, the mass does not add up to that of the original atom.
The lost mass is around 0.1% of the original atom’s mass. This small amount of matter is literally converted from mass to energy. If you’ve ever heard of Einstein’s equation E = mc^(2), then you know that energy equals mass times the speed of light squared. The amount of energy you get from that mass-to-matter conversion follows this formula. In that formula:
Energy is Joules
Mass is kilograms
C is a physics constant for the speed of light in a vacuum, or 299792458 m/s.
So if you convert 1 gram (0.001 kg) of matter to energy, you’d get:
E = 0.001 × 299,792,458^(2)
Which works out to 89,875,518 Megajoules (8.9875517874×10^(13) Joules), or roughly 24,965,422 kWh. For a sense of perspective, a Tesla Model S P100D has a 100kWh battery. According to the EIA, the average US residential customer used 867 kWh per month in 2017. So almost 25 million kWh of energy is a LOT of energy, and that’s just in 1 tiny gram of matter.
In reality, we aren’t able to convert 100% of that energy into electricity. The conversion of heat to electricity in a typical nuclear power plant is just under 40%, so less than half of that is converted to energy. That’s still a lot of energy though.
As for why mass converts to energy at this specific rate, this is one of those fundamental questions that doesn’t have a more detailed answer than, “because that’s how it works.”
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