# Unstable nuclei

1. Oct 29, 2014

### Fabian901

I've started studying physics at a basic level and I've ready that beta decay takes place when the number of neutrons exceed the number of protons in a nucleus. Why does this excess number of neutrons compared to protons make the nucleus unstable?
Also, what does an unstable nucleus mean?

2. Oct 29, 2014

### Staff: Mentor

Unstable means there is a probability the nucleus will spontaneously split into smaller parts.

Nature prefers lower energy states. The energy of larger nucleus is higher than smaller ones so they will try to spilt to go to a lower energy state.

Exactly why it does this theoretically I don't know - nuclear physics is not really something I am up on.

But I suspect it's related to why excited electrons spontaneously emits photons to go to lower energy states - its tied up with Quantum Field Theory and the vacuum. Vacuum fluctuations are responsible - my suspicion its its the same with spontaneous nuclear splitting.

Thanks
Bill

3. Oct 29, 2014

### Fabian901

Thanks a lot for the answer! Where does that energy in the nucleus actually come from?
Many thanks!

4. Oct 29, 2014

### mathman

Not quite true. For increasing atomic numbers stability requires more neutrons than protons. If the ratio is too large or too small, there will be decay.

Heaviest stable nuclides are those of lead, 82 protons and 124 to 126 neutrons.

5. Oct 29, 2014

### Staff: Mentor

Imagine that the protons and neutrons fill up two "parallel" sets of energy levels. For lower proton numbers the energy levels are nearly the same, so for a given total number of nucleons you get the lowest energy when you have half neutrons and half protons. See the picture associated with the asymmetry term in the Wiki article on the semi-empirical mass formula:

http://en.wikipedia.org/wiki/Semi-empirical_mass_formula#Asymmetry_term

For larger proton numbers, the electrostatic repulsion "spreads" the proton energy levels upwards and further apart, so the lowest-energy configuration has fewer protons than neutrons.

6. Oct 31, 2014

### Fabian901

I see, thanks a lot for the answers!