Why do some elements not have a stable isotope?

Question

My (extremely limited) understanding of what causes an element to be either stable or reactive is due to a mismatch in energy in the nucleus due to either extra protons or extra neutrons. So why wouldn’t something like Plutonium-188 be stable, since it would have 94 protons and 94 neutrons?

In: 5

Answer 1

Its probably not an ELI5 answer, but here we go. It’s all a careful balancing act of the electromagnetic force, the weak nuclear force, and the strong nuclear force. The electromagnetic force you should be familiar with, charge and magnetism. We are really only concerned with charge here. The weak force is what’s responsible for beta decay, and the strong nuclear force is the main thing holding the nucleus together.

The electromagnetic force means that the protons in the nucleus are going to repel each other. The strong nuclear force at close distances (less than 1.7 femtometers) so the can be held together. So if a nucleus is too big, it will push itself apart with its own electrostatic repulsion. That’s why a lot of larger elements are radioactive. Alpha decay and proton ejection are both a result of this happening.

Then why aren’t super small nuclei a thing? Why don’t we have helium-2, but rather helium-4 and small amounts of helium-3? Well that’s where the weak nuclear force comes in. If we did have helium-2, it would β+ decay into deuterium (hydrogen-2). Now I’ll be honest, the weak force is the one I understand the least, but essentially it prevents the overcrowding of protons or neutrons, but basically in the helium-2, the protons are overcrowded, so it turns one into a neutron to restore balance. The same would happen in helium-6 with too many nuetrons, but would undergo beta decay instead of β+.

Plutonium-188 would likely rip itself apart in a fission reaction thanks to the electromagnetic force, but if it didn’t it would almost certainly β+ decay into something more stable.

To really mess with your brain, we cause fission of uranium-235 by adding a neutron, the resulting uranium-236 undergoes fission, but uranium-238 is not only (relatively) stable, but the most common form of uranium.

I hope this helps.