Stablity of the nucleus

[scope]

hi,

I am wondering why even numbers of protons, or neutrons, or electrons, are more stable that odd numbers?

i have read this on wikipedia:

http://en.wikipedia.org/wiki/Oddo-Harkins_rule

" In elements with even atomic numbers, protons are paired, with each member of the pair offsetting the spin of the other, enhancing stability "

the same principle applies for electrons?, so I was wondering what is the cause for this? I believe this is related to magnetic moment that leads to a magnetic attraction, or i am wrong?
please reply!

 

[bcrowell]

It applies to protons, and it also applies to neutrons, but it doesn't apply to electrons. It arises from the short-range, attractive nature of the nuclear force. Since the force between electrons is long-range and repulsive, you don't get the same effect.

The basic idea is that when you have a short-range, attractive force between, say, two neutrons, stability is optimized when the two wavefunctions are the same except that one is time-reversed compared to the other. Classically, this would be like having the two neutrons occupying the same orbit, but going around in opposite directions. Because the orbital planes are lined up, you maximize the overlap between the two wavefunctions, which means you're maximizing the binding.

 

[scope]

then why unpaired electrons are more reactive than paired electrons? this seems to mean that they are less stable

 

[bcrowell]

then why unpaired electrons are more reactive than paired electrons? this seems to mean that they are less stable

The signature of pairing in nuclei is that when you plot the binding energy as a function of N for fixed Z, or as a function of Z for fixed N, you get an odd-even staggering, like a sawtooth. There is no such odd-even staggering for electronic binding energies: http://en.wikipedia.org/wiki/File:IonizationEnergyAtomicWeight.PNG