The nucleus of an atom is composed of neutral and positively charged particles. Wouldn’t the protons repel each other enough that they would fly apart? What is holding it together?
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The ELI5 answer. As strong as the nucleus wants to push apart an even stronger force is keeping it together. It’s called the Strong Force. Appropriately named.
The strong nuclear force. It’s one of the four (three) fundamental forces of the universe, and it is only attractive. It’s what holds quarks and nuclei together, and it’s about 100 times stronger than electromagnetism.
You’re right that the force of *electromagnetism* tries really hard to push those protons apart. But there’s another force at play here, called the *nuclear force* or the *strong nuclear force*, holds the protons together even more strongly than electromagnetism pushes them apart. But it only works at incredibly tiny ranges – the distance between atoms is so great that they can’t affect each other, but within a nucleus itself, the strong force is king.
Sort of like how at on our human scale, electromagnetism can be stronger than gravity. We can use magnets to lift heavy metal objects, even thought the force of gravity wants to pull them down – one force is overcoming the other. But at long ranges (like the distances in space), gravity is the only force that still holds power, and it controls the interactions between the planets. Just like how electromagnetism can beat gravity at the human scale, the strong nuclear force can beat electromagnetism at the tiny scale.
There are opposing forces.
The electromagnetic force you allude to holds negatively charged electrons in their orbit around the nucleus, which contains one or more positively charged protons.
The nucleus itself, however — protons and neutrons — would fly apart because of the electromagnetic force since their charges are all positive or neutral. However, they do not fall apart because a fundamental force called the **nuclear force** holds them together. The nuclear force is probably four times as powerful as the electromagnetic force so it wins.
Gluons are holding it together, which are the force particles that mediate the strong nuclear force.
As the name suggests, the strong nuclear force is… strong. However, it’s also very short-ranged. Its strength falls off very quickly as particles get further apart. And this imposes a limit on how big a nucleus can be, which is part of the cause of radioactivity. Beyond a certain size, the strong nuclear force isn’t strong enough to keep the nucleus together, and the forces that want to rip the nucleus apart (of which electromagnetism is one) become strong enough, and the nucleus does in fact fall apart. When that happens, we call it radioactive decay.
As others have said, there is a short-ranged and attractive “strong force” that is much more powerful than the repulsive force of electromagnetism that you are referencing when talking about the protons repelling. An interesting corollary: what if you had an atomic nucleus that was so large that it was almost the range of the strong force, and you stretched it along one axis? At the right amount of “stretching,” the nucleus would behave like two separate nuclei right next to each other, and repel with great violence. This is what happens with nuclear fission, and is why you don’t have to “smash” heavy atoms apart, you can just sort of jiggle them with a neutron.
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