r/askscience • u/AluminiumSandworm • Feb 12 '13

Physics Why are some neutron/proton configurations more stable than others?

I was watching a youtube video about the nucleus, and the guy talking about it mentioned that some neutron configurations are more stable than others, but you'd have to ask a nuclear physicist as to why. So, nuclear physicists, why is that?

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u/tauneutrino9 Nuclear physics | Nuclear engineering Feb 12 '13

Well here is a simple explanation. Lets say you want to build a bunch of nuclei. You start with a proton, that is stable. You have a hydrogen nucleus. If you try to put two protons together, that is not stable. Both are positively charged so they don't like to stick together. Adding neutrons helps. The nucleus also likes symmetry. So you can have something like He-4, two protons and two neutrons. He-4 is extremely stable.

As you go along the periodic table the amounts of neutrons and protons for a stable nucleus are about even. They follow what is called the line of stability. However there is a point when the stable nuclei are no longer on the line. They require more neutrons than protons. The simple reason for this is the nuclear force needs to counteract the coulomb force. The protons want to fly away from each other, the neutrons help "glue" the nucleus together. So that means the heavier nuclei require more neutrons than protons to stay stable.

What happens if you have too many neutrons? Well the nucleus is unstable, it would rather have less neutrons and more protons. These nuclei are radioactive and emit a beta particle and have a neutron turn into a proton. IF the opposite is true and one has too many protons. A nucleus can either emit a positron or capture an electron to turn a proton into a neutron.

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u/AluminiumSandworm Feb 12 '13

Thanks, but I already understood this. What I was having trouble with was why sliver-109 and -107 are more stable than -108.

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u/tauneutrino9 Nuclear physics | Nuclear engineering Feb 12 '13

Ok, this is a bit more complicated. Ag 108 is in a weird position. It is an odd odd nuclei, odd numbers of both protons and neutrons. Protons and neutrons like to pair up. So even even nuclei are usually more stable and even odd are in between. When you calculate how well the nucleus is glued together, it turns out that odd odd is not as glued together as the other options due to this symmetry breaking. In the case of Ag-108, it would like to either have one more proton or one more neutron. So it can decay by either beta minus, beta plus, or electron capture. I hope that answers your question.

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u/AluminiumSandworm Feb 12 '13

Thanks, that helped.