Neutron Configurations: Why Certain Atoms are More Stable

[scott_alexsk]

Does anyone know if there are descriptions of the neutron and proton configurations in atoms and why certain configurations are more stable than others.

For example the average atomic mass of oxygen and flourine are the whole numbers 16 and 19 while most other elements have more variation. Is it just that 8 neutrons with 8 protons is the only configuration for 8 protons that can exist? Or is it that any other configuration for oxygen is so minute and unstable that it has not been found? Why should other configurations for say Aluiminium with 13 protons have more variance in the neutron number?

Thanks,
-scott

 

[chroot]

According to Wikipedia, oxygen has three stable isotopes. Virtually all the oxygen on Earth is oxygen-16, which is why the average mass number printed on your periodic table appears to be a whole number. It's not really a whole number, it's just that it's a whole number within the precision of at least two or three decimal places, as printed on your table.

http://en.wikipedia.org/wiki/Oxygen

- Warren

 

[scott_alexsk]

Yes but also according to Wikipedia flourine has only one isotope. So the question still stands.

Thanks,
-scott

 

[chroot]

Fluorine has only one stable isotope -- the rest are radioactive, most with rather short half-lives, and thus are not found in nature in any meaningful quantities at any given time. Remember that the average mass printed on the periodic table is the average mass of the isotopes that are found, naturally-occuring, on Earth.

http://en.wikipedia.org/wiki/Isotopes_of_fluorine

- Warren

 

[Astronuc]

The stability has to do with binding energy -
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html

And internally the pairing of protons/neutrons. Nuclei with the same number of neutrons and protons, or a slight excess of neutrons are somewhat more stable. But this is a general rule, and there are exceptions.

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html

See - Shell model of the nucleus (especially the second plate)
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/shell.html

 

[scott_alexsk]

So are they comparing the electron proton structure to the interaction between protons and neutrons? So I suppose as an electron proton configuration has certain stable designs varying neutron and proton configurations also have certain stable designs. I have to ask, would the protons be orbiting the neutrons or the neutrons the protons, or am I way off and should stop asking?

-scott

 

[chroot]

I don't know what an "electron proton structure" is. Electrons exist far, far outside the nucleus, and don't have (much) to do with a nucleus' stability, with the exception of decay modes like electron capture.

The neutrons and protons do not orbit one another in any classical sense; remember that subatomic particles (including electrons) do not really orbit in neat little circles. Instead, they are spread out with a "fuzzy" probability density.

The location of any particular proton or neutron in the nucleus at any particular time is given by such a probability density. Note also that quantum mechanics forbids "tagging" one proton and watching it move independently of the others anyway; two protons are physically indistinguishable particles.

- Warren

 

[scott_alexsk]

Yes I know, sorry for my attachment to classical depictions. Let me rephrase it. Will the proton cloud be intermengiled with the neutron cloud or will they be two separate descrete entities such as in the relation of the electron's cloud to the nucleus/proton cloud?

Thanks,
-scott

 

[chroot]

The probability density functions of the protons certainly overlap those of the neutrons. This doesn't mean, however, that any given protons and neutrons actually "coexist" in the same space at any given time.

- Warren

 

[scott_alexsk]

Let me phrase it this way then, the typical laymen's description of the nucleus is that the neutrons and protons are intermixed without any logical pattern and collide with each other constantly. Now the link that astronauc posted suggests that the neutrons and protons have a sort of shell structure as electrons do with the nucleus. My question is, are the protons on the inside or outside of the nucleus? My guess would be outside. Is there experimental data that can prove that the nucleus has a shell structure besides patterns that have been observed indirectly?

Thanks,
-scott

-scott