how can splitting a tiny piece of an even tinier atom create such a vast explosion?


how can splitting a tiny piece of an even tinier atom create such a vast explosion?

In: Physics

The bonds that hold atoms together is incredibly powerful. Breaking that bond releases energy.

The best ELI5 analogy would be a spring. If you had a powerful spring and a force kept that compressed until the force was removed, all that potential energy is released. Except the spring is really tiny and really confusing physics keeps is compressed.

Even though they are tiny, the amount of energy holding those atoms together is really strong (relatively). When the atom is split that energy is released. Lots of atoms get split in an almost instant chain reaction, lots of energy gets released, and boom!

If you have a small piece of uranium, it decays by itself and releases small bursts of energy. That energy is powerful for its size, but it’s not explosive and would not really hurt you unless you spend time with the uranium. And even then, it is cell damage and cancer risk that will hurt you, not an explosion.

When you get to a nuclear reactor, you have a lot of radioactive material with the impurities reduced. The energy released from one of the radioactive atoms causes the atom near it to release its energy. If one atom causes one other atom to release it’s energy, you have a sustained reaction. And one of the side effects is decay heat, which can be harnessed for energy.

Now a reactor can become explosive when one atom sets off more than 1 other atom. Those atoms set off multiple other atoms and now, you have a dramatic increase in heat. Heat causes liquids to turn to gas and causes gases to expand which can cause an explosion. But even then, it’s not really a nuclear explosion per se… It’s an explosion caused by heat build up. (The heat can also set things on fire but those also aren’t really nuclear explosions either. Additionally, nuclear reactors can trigger chemical explosions, like splitting water into hydrogen, which goes boom, but that’s not a nuclear explosion either.)

An intentional nuclear blast is an even more extreme version of this. It requires momentarily packing so many radioactive atoms next to each other in a high energy environment that many of the atoms release their energy all at once: that is, the chain reaction of atoms starts with many atoms going off and they set off many other atoms all at once (like the run away reactor but with a huge starting number of atoms and huge number of nearby atoms set-off by each atom.)

But the main point is the power of a single atom is not explosive per se. Nuclear bombs can only become explosive because many atoms are releasing their bonding energy at once in a chain reaction.

You don’t need to actually know math, I promise. Bear with me:

The relevant equation is

E=mc^2 (yes, *that* E = mc^2 )

Energy = mass x the speed of light *squared*

Splitting atoms in a nuclear explosion converts a tiny bit of matter (mass) into energy. But the speed of light is a huge number. The speed of light *squared* is so utterly gigantic that multiplying a small mass by this HUGE amount means you release a ton of energy in a very small space – aka a vast explosion.

This answer brought to you by a man. That man’s name? Albert Einstein.

How much energy does it take to use a knife to cut a ratchet strap under a LOT of tension? (very small)

How much energy does it release? (very big)

The amount of energy contained in each molecule is by proportion tiny, but by scale, massive

Take 1 uranium 235 molecule. Break it apart, it sends out a tiny pop.

That tiny pop triggers other uranium around it to break up, releasing their tiny pops

Take 235 grams or about half a pound of a certain kind of uranium, let those molecules pop, that adds up to what’s called avagadro’s number of molecule pops. Avagadro’s number is 6×10^23. Boom

The thing here is it is the chain reaction that your missing. While splitting one individual atom does not release a lot of energy, the splitting of that atom splits two more. That splits 8, 16, 32, 64, 128, 256, 532, 1064, 2128. This all happens in a fraction of a second and as you can see ten reactions later we can now splitting two thousands of atoms at a time. In ten more reactions we will be splitting two million atoms at a time.

If we can control the reaction so each atom only splits one other atom we have a very good source of power. But if it splits two instead of one it can quickly get out of control and explode.

On a related note I’d also like to know why the inverse is true for lighter elements like fusion for hydrogen. Why does THAT *release* so much energy when we require to input a lot of energy to fuse atoms??

It doesn’t. Splitting an atom releases some energy and some free neutrons. Those free neutrons can then go and split more atoms. This is what powers a nuclear power plant. You have a slow, controlled reaction to produce heat. The nuclear fuel is slowly used up over a period of years.

A bomb tries to split as many atoms as it can at once. The combined energy of the fission of all of those atoms at once is what produces such a massive explosion.

When you cut through something with a knife (an onion, piece of paper, lump of lithium, a log, whatever), aren’t you sometimes purely by chance slicing through atoms? Why isn’t there a reaction when splitting these atoms?

It doesn’t. There are lots of atoms.

Like, a LOT. You may think there are a lot, but then you look it up, and it’s way bigger than you thought.

It doesn’t. The explosion happens when you split lots and lots and lots of atoms at the same time. Just like a gasoline fire means lots and lots and lots of molecules have to burn. But you still get way more energy per atom than you get with burning.

To add on to the other comments, it’s also *hard* to get that explosion. If atoms are releasing a ton of energy, the atoms near them tend to be blown away from each other. That makes them too far apart to actually slam into each other in order to get them to split. As a result, you have to have a *massive* system in place to try and prevent that as long as possible. The Fat Man bomb had 6.2kg of plutonium, surrounded by over 4500kg of high explosives and a few other tricks in order to crush the plutonium atoms together long enough for them to split. It successfully kept the plutonium atoms close enough together for long enough that enough that about 1kg of the 6.2kg in the bomb actually split. To give a sense of scale here, solids are effectively uncompressable in everyday experience, but in this case a roughly soda can-sized chunk of plutonium was crushed down to the size of a chicken egg for a duration of a few microseconds. For a bomb like Fat Man, even a slight error in the timing of the explosives for compression, microseconds or fractions of microseconds, can result in the high explosives trying to crunch the plutonium having a bigger boom than the plutonium itself does. It requires *incredibly* precise conditions.

To give another sense of scale here, it’s not just a few atoms splitting. If I did my decimals correctly, in the Fat Man bomb, it was about as many plutonium atoms as there are stars in the observable universe, or the number of grains of sand on 1000 copies of earth, that all split in a few microseconds.

Modern fission weapons use various methods so that they can get by with a much smaller amount of explosives around it, but the principle is the same.

Something that I think is not emphasized enough on other replies is that in the explosion of an atomic bomb you are involving millions of millions of millions of millions of atoms (around 6^24 ). This is an incomprehensibly big number.

Eli5:. Think of an atom like a classic mousetrap.

The arm that you press down and latch is analogous to creating the atom in the first place.

The “arm” is stiff and resists you pressing it to the base. You have to apply a large, constant force to press it into place.

This happens in atoms because protons really push hard away from other protons (like electrical charges repel). But with enough force from outside, you can squeeze then together. This force for atoms comes from the intense heat and pressure found in the coe of stars, or during supernovas.

Once the arm is down, there is a latch that holds it in place. This latch is very short range, it only works once you get the arm very close to the base. This latch is also much stronger than the arm, since it holds it in place.

In an atom, this is the role of the strong nuclear force. It is an incredibly strong, but very short range force. If the protons are pushed into the region where it is stronger than the electric force, they will snap together, despite the protons pushing apart.

For fusion energy, stars push together small atoms, like hydrogen and helium. When the string force takes over, there is plenty of room left for them to move when they snap together. This means the atom sorta “jumps” when they collide. This can bump nearby atoms and transfer the energy away as heat.

For atoms larger than iron, there really isn’t enough space for then to snap together. It’s more like you’re trying to cram more and more stuff under the latch that is the strong force.

If you smack the latch of a mouse trap, then the spring arm flies free. Same with an atom that experiences fission. If you smack a large atom a part might fly free, propelled by the protons hating other protons, and the rest of the atom crunching closer under the strength of the strong force.

Follow-up question: how do you actually break atom bonds?

Imagine there’s layers like an onion to get to the middle of the tiniest weapon of mass destruction. Ones you get to the core, there’s only one layer left until it explodes. By leaving the forbidden grape alone, we can avoid massove boom-booms.

A single atom? The energy is tiny. But way more than combining a pair of hydrogens with an oxygen one for example.

A uranium rod has many atoms, many of which are splitting all the time.

Depending on how close your splitting atom is to others it might only make a Lil heat, or hitting another atom will destabilise it enough to split it.
In a power station you balance this for a steady elevated rate. In a bomb you aim to split as many as you can in a fraction of a second

because even tinier little bits got turned completely into energy. Kind of like how a little bit of your eraser rips off, but instead of balling up it’s blasting through solid surfaces.

But there are no tiny pieces of atoms that are paradoxically bigger than the atom itself.

Interlock your fingers together and don’t let go. Try to pull your hands apart. Your grip is likely stronger then your ability to pull your hands apart. Now imaging Dwayne the Rock Johnson walks up and pulls your hands apart. He’s pulling and pulling and your grip is strong, but you can feel your fingers weaken. Suddenly your fingers lose grip and your hands fly apart in a burst of energy.

That’s what happens when you split an atom. Energy used to keep bits together is released. Except the energy released is enough to release more bits. And in an instant, protons are flying in all directions releasing ever more energy until they run out of other protons to hit.