Re: [Phys-L] Energy & Bonds

[Bill Nettles <bnettles@uu.edu>]

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Bruce
> Sherwood
> Sent: Thursday, November 14, 2013 3:33 PM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] Energy & Bonds
>
> The nuclear shell model is a nice example implying the existence of well-
> defined nucleons in a nucleus.

So the shell model of atoms with electrons implies the existence of well-defined electrons?  No, just the existence of a cloud of electrons which have distinct groupings based on binding energy and angular momentum.  The cloud has Z (or Z-1, etc) -e charges and we can recreate an individual electron if we put in enough energy.  But you can't get that electron as a separate entity until you give the mass-energy back, just as in the deuteron, you cannot get a separate proton or neutron out until you give the mass-energy back.  In chemistry the mass-energy deficit is small, but it's still there. It's just not practical for compound calculations.  On the other hand, it totally explains the binding.

I agree, there are appropriate contexts in which to talk about O=O being different from 2 O and that being different from O-3, but shouldn't chemistry be the place to do it.  Why NOT say they are different? I also agree that we should understand what the building blocks of molecules and nuclei are, but when we put them together, they behave like something new.

The nuclear shell model is useful for discussing the behavior near magic numbers, but it's not the be all/end all of models. That the shell model works (in some regimes) should not imply  that nucleons exist as well-defined particles in the nucleus. The nucleus is not a bag of marbles, formed from constituents which are no longer individualized.  When you are between magic numbers above A=50-60, the shell model doesn't work well.  

That being said, the constituents are thought of as particles with interactions, and we can make predictions based on the type of particle makes us the constituents.  Adding a neutron to a nuclide (neutron bombardment) will tend to create a beta-decaying nuclide, but not always.  Having an (n,p) or (p,2p) reaction might create a positron emitter or electron-capture nuclide, but not always.  The particles in the nucleus (and the atoms in a molecule) do NOT behave as individuals in the majority of their interactions with outside entities.  And you have to do some special energetics to look at individuals. A deuteron behaves very different from a proton and a neutron.
 
It's kind of weird when adding 2 neutrons to a nuclide far from stability changes its charge distribution shape radically, from prolate to oblate.  That's what I studied as an experimental nuclear physicist.  Yeah, I still talk about the number of protons and neutrons in the nucleus, but I say that they don't exist well-defined, and I can't localize them. And adding/removing one proton or neutron dramatically changes properties of the system.

Pairs of like nucleons do seem to combine angular momenta to get zero (all even-even nuclei have a ground state spin of zero, with positive parity).  That indicates to me that they interact strongly in pairs.
> In this context I'm not sure what the message is, but note the apparent
> existence of alpha particles in nuclei, as seen in alpha decay (but on the other
> hand you don't see proton or neutron or deuteron or triton decay).

The Gamow model of alpha decay indicates that a proto-alpha can form and then tunnel out. Clumps of alphas seem to be preferred. Hmmm. And why is He-3 stable?