And if splitting atom releases energy, why haven’t these energy break from their atom themselves? Isn’t that means the force that bind the atoms are bigger than the energy released?
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A single fission reaction will release some energy, but not enough for any sort of explosion. For a nuclear bomb we want to have a lot of fission reactions in a short period of time.
One way of causing a fission reaction is to take the fissile material and hit it with a neutron. Now the handy thing here is that some fission reactions also release neutrons, and so you can try and get those neutrons to cause even more fission reactions. This is called a chain reaction. When the conditions are right for the chain reaction to continue happening without us having to add more neutrons, we say that it has reached criticality.
Criticality is affected by multiple factors, such as what the fissile material is, how much of it there is, how pure it is, what it’s surroundings are, etc. One thing we do to reach criticality is surround the fissile material with neutron reflectors, these are materials which reflect neutrons back towards the fissile material so as to increase the chance of them hitting the fissile material to cause a reaction. Another thing we do is refine the ore to get the specific elements that are fissile in a high enough concentration.
Without this refining and use of neutron reflectors, there isn’t anything to cause the ore to reach criticality.
With nuclear weapons (as opposed to nuclear reactors), you also want it to happen suddenly, so you need to start off with something that isn’t critical and suddenly make it far beyond critical. Taking the two atomic bombs of WWII as an example, one achieved this by taking two sub-critical masses and firing them into each other, while the other took a sub-critical mass and changed the criticality by increasing the pressure using explosives to create an implosion.
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