Eli5: Why does splitting an atom cause such a giant explosion?


Where does all the energy come from?

In: 10

E=mc^2. Fission converts a bit of mass to energy. Though the mass is small, the c^2 factor is huuuuuge. Also, in a fission bomb, you are splitting many many atoms.

Splitting a *lot* of atoms releases a lot of energy.

Just like burning one molecule of gasoline does nothing, splitting one atom of uranium does nothing.

Burning a gallon of gasoline releases a lot of energy. Splitting thirty pounds of uranium will destroy a town.

The energy comes from forces within the nucleus. One force holds it together, while another tried to push it apart. By splitting it, we allow that repulsive force to violently fling the nucleus apart, and release a ton of energy.

Splitting *an* atom does not release all that much energy. It’s just enough to produce a visible flash (which is already a huge amount of energy for an atom).

But there are a *lot* of atoms in even a small piece of material, and a nuclear bomb causes a significant percentage of *all* of those atoms to split within a fraction of a second.

When atoms were born they were all the same, hydrogen atoms, one proton, one electron… that’s it.

But they had gravity and they grouped together forming stars. A star is when all this atoms group in a gigantic ball, but the ball gets too big, the pressure inside compress the hydrogen and they fuse together. This makes a lot of energy. This energy heats all the star and a billion year fusion reaction starts. Inside, it’s a gigantic mess. Atoms love to get bigger, but not too big. They keep fusing one into another forming bigger and bigger boring atoms like iron. But some atoms by chance are fused with other ones too much and that makes big angry atoms like uranium, a very big atom with too many protons and neutrons inside. Protons and neutrons don’t like to be pressed together in so little space and they want to separate, but they don’t have enough force, just a little too weak to divide.

Then the star explodes and shoots all this atoms around the space.

Some days later, actually not days, billions of billions of day, this atoms are again together because of gravity, and form a planet. A planet with water. And this planet makes animals and some are intelligent and at a certain point a scientist figures out that atoms love to get big but not too big. And had the idea to take the very big atoms and give them a chance to separate into smaller ones.

When they separate they release part of the gigantic energy that was shot into them when they were made.

Idk why atoms would love to be iron (they like to fuse together to make it or split to make it) most will never be iron, but they want to. But anyway it’s a fact, I just don’t know why.

It turns out that an atoms don’t split quite evenly. In a nuclear reactor, a uranium atom decays into a thorium atom and a helium atom (roughly speaking). Uranium has mass 235.0439299, thorium has mass 231.03630, and the helium atom has mass 4.001506466. If you add all those up, though, you’ll notice that about 0.0061 mass is missing. What happened to it?

Atoms have very specific mass amounts, so we can’t just tack the extra onto one of the two new atoms. Instead, the leftover mass gets directly converted to energy according to the famous e=mc^(2) equation. Since the c^(2) factor is massive (roundabouts 18 zeros if you’re using meters per second), even the tiny amount of mass converted adds up *extremely* quickly. It adds up even faster when you consider that you aren’t just doing this to one atom, you’re doing it to a whole bomb or reactor core’s worth at a time and atoms are REALLY tiny (add another 20+ zeros)


Disclaimer: I got the exact numbers from a quick google search, so they might not be entirely accurate, but the process is.