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This is a great response! Now do the 'why' it decays, that will surely be the follow up question!
I only didn't ask this because Sagan actually explains that part in the book.
If I understood correctly, protons repel each other, and the reason atoms don't usually tear apart is because neutrons exert a force over protons that can counter that repelling and keep the atom's nucleous together. If an isotope does not have enough neutrons to fully counter the proton's repelling forces, it is deemed "unstable" and eventually loses nuclear particles. This is the case of Uranium-238.
Why uranium-238 loses specifically 2 protons and 2 neutrons was not explained, but I assume it's a result of the specific difference between the forces of neutrons holding it together and the forces of protons repelling each other.
Feel free to correct me if I got anything wrong.
Sorry for the late reply, I love geeking out on this. Please note that some of this is off the top of my head coupled with online research and online lectures.
You have a solid understanding of why atoms decay, and you’ve got the big picture right. The nucleus is held together by the strong nuclear force which binds protons and neutrons together. But protons are all positively charged, so they are constantly repelling each other due to the electromagnetic force. Neutrons help by adding extra strong nuclear force without adding more repelling charges, but if an atom has too many or too few neutrons, the balance starts to break down. That’s when an atom becomes unstable and eventually undergoes radioactive decay.
Uranium-238 specifically loses 2 protons and 2 neutrons as an alpha particle instead of, say, just a single proton or neutron due to energy and stability. Large nuclei like uranium are constantly vibrating due to the competing forces inside. Over time, quantum mechanics allows certain particles to "tunnel" out of the nucleus, meaning they escape even though they don’t have enough classical energy to do so. Alpha decay happens because helium-4 is an extremely stable nucleus, so when the uranium nucleus reaches a state where part of it can separate, it naturally forms this tiny, tightly-bound chunk rather than spitting out individual protons or neutrons.
Another factor is that heavier elements tend to decay in ways that release as much energy as possible while still leaving a stable nucleus behind. If uranium-238 lost just one proton or neutron at a time, it would still be unstable and just keep decaying step by step. But losing an alpha particle is a more efficient way for it to drop to a lower-energy, more stable state quickly. Thorium-234, the element left behind after uranium-238 decays, is still radioactive, but it is slightly more stable than uranium, so this decay step makes sense in the long run.
So uranium-238 decays via alpha emission because it is the most energetically favorable way for it to become slightly more stable. The strong nuclear force holds nuclei together, but once an isotope gets too large and the balance between attraction and repulsion is off, the nucleus finds a way to shed some mass to stabilize itself. The reason why it does so by specifically ejecting a helium nucleus rather than some other particle comes down to quantum mechanics and the fact that helium-4 is an incredibly stable arrangement of protons and neutrons.
Also, thank you for all of the wonderful comments!
Once again a great response, thank you so much! You really are restoring my faith in social networks.
I am curious now, are there isotopes of uranium or other elements that release different particles? Like a hydrogen nucleous?