Why do atoms undergo alpha and beta decay?

[LotusTK]

My understanding of what makes an atom unstable is this:

Large nuclei will have a high number of protons, and as a result there will be large amount of electrostatic repulsion between the protons in the nucleus. So when the nucleus is too large (too many protons) The electrostatic repulsive forces between the protons is greater than the strong nuclear forces that are acting between quarks in neighboring nucleons. (The strong force is obviously strong in each individual nucleon, but is less strong between the quarks which are not in the same nucleon)

So when an atom needs to become more stable, it needs to reduce the size of its nucleus, and it can do this via alpha decay. Or it can reduce the repulsive forces present in the nucleus by undergoing beta plus decay, which changes an up quark to a down quark, and as a result turns a proton into a neutron.

(But Beta minus decay turns a neutron to proton, so isn't that particular decay a step towards instability?)

Would be grateful if someone could make this clear for me. Thanks in advance.

 

[mfb]

That description is fine.

(But Beta minus decay turns a neutron to proton, so isn't that particular decay a step towards instability?)

A nucleus has separate energy levels for protons and neutrons, and they get filled starting from the lowest energy levels. If you have too many neutrons compared to protons, you have to fill quite high-energetic neutron states, while proton states at lower energy stay unfilled. That makes beta- decay possible - a neutron converts to a proton, which releases energy (and an electron and an antineutrino).

 

[LotusTK]

Thanks for the reply.

I didnt know protons and neutrons had energy levels, i thought that was only for electrons.

So would i be correct in saying that the two types of beta decays control the neutron proton ratio?

And by looking at a neutron to proton graph which shows "regions" of stability and instability, stable nuclei have more neutrons than protons, which makes sense because the strong nuclear force argument i mentioned above. But having too many neutrons will make the atom unstable as it will be vulnerable to beta minus decay. And the reason why you can have too many neutrons is because of those energy levels you mentioned? I suppose the same is also true for protons, but you can also say that its worse to have more protons in terms of instability because of the electrostatic repulsion between the protons.

 

[mfb]

Right.