Central Problem

[abacus9900]

If the nucleus of an atom consists of protons and neutrons and neutrons have no charge, while protons repel one another, how come the nucleus does not disintegrate?

[principled]

Abacus,
Whilst travelling back to the UK last night I was listening to a podcast from April last year. The contributors were discussing new elements 117 Ununseptium and 118 Ununoctium that have been "discovered" (see link below ).
During the discussion they talked about the fact that most "fabricated" elements are unstable and decay very quickly, but the discovery of those above gives credence to the hypothesis from the 70s of an "island of stability" in terms of heavier elements.

It would be good to know what lies behind this hypothesis, because it must also be linked to the stability of the nucleus as per your OP.
www.radiochemistry.org/periodictable/elements/117.html

[Progenitor A]

Mar 11, 2011 15:09:04 GMT 1 abacus9900 said:

If the nucleus of an atom consists of protons and neutrons and neutrons have no charge, while protons repel one another, how come the nucleus does not disintegrate?

Strong nuclear attractive force overcomes electric repulsive force between protons
Strong nuclear attractive force keeps atomic nucleii together

Strong nuclear attractive force acts only over very small distances

That's me, my nuclear knowledge exhauste

[abacus9900]

Yes principled it does seem to relate to my question but we await some answers.

[abacus9900]

Mar 11, 2011 17:32:32 GMT 1 Progenitor A said:

Mar 11, 2011 15:09:04 GMT 1 abacus9900 said:

If the nucleus of an atom consists of protons and neutrons and neutrons have no charge, while protons repel one another, how come the nucleus does not disintegrate?

Strong nuclear attractive force overcomes electric repulsive force between protons
Strong nuclear attractive force keeps atomic nucleii together

Strong nuclear attractive force acts only over very small distances

That's me, my nuclear knowledge exhauste

Ah, so that's it. Many thanks.

[speakertoanimals]

The strong nuclear force is actually quite complicated, and is a residue of the fundamental strong force that holds quarks together inside nucleons (neutrons and protons).

Think of the electromagnetic force -- this holds electrons inside atoms. The atom as a whole is electrically neutral, but a residue of the em forces acts between neutral atoms in molecules (the van der waal forces). Similarly, a residue of the strong force acts between nucleons, to bind nuclei togther.

Just as the em force within atoms binds electrons to the nucleus, and gives a series of atomic electron energy levels, so the strong force within the nucleus gives a series of neutron and proton energy levels, to give the shell-model of the nucleus, a bit like the shell-model of the atom that explains chemistry. But the nucleus isn't as simple as the em forces within an atom, hence nuclear physics is a bit more complicated than atomic physics.

So in some circumstances, nuclei can behave like droplets of liquid, hence the liquid drop nuclear model. The aim is to try and explain the patterns we see in terms of nuclear binding energy, why some nuclei are more stable than others, hence why nuclear decays occur in the way they do.

So that gives us now the four fundamental forces -- gravity, which holds planets bound in solar systems, but is negligible on anything less than planetary scales. Then we have electromagnetism, which holds electrons in atoms, and atoms within molecules, and explains chemistry and the way matter holds together.

Then we have the strong nuclear force, which is smaller-scale again, holds nucleons in the nucleus, and quarks within nucleons.

The final force is the weak force, which is too weak to hold any systems together, and just appears in certain nuclear decays.

In terms of the fundamental force carriers, we have the graviton for gravity, and the photon for em, both massless, hence these are both long-range 1/r^2 forces. The force carrier for the strong force is the gluon, which although theoretically massless, unlike the photon (which is not electrically charged), the gluon feels the colour force as well as mediating it.

Finally, the mediating particle for the weak force are the famous W and Z bosons, which are massive, explaining why the weak force is so weak!

Putting three of these together (strong or color force, em, weak) is what the standard model does.

[Joanne Byers]

I am already tired of editing out STA's b*ll*cks, idi*ts and f*cks.

So I'm stopping here.

If you continue to pepper your posts with such language, STA, you will be banned. That's a PROMISE, because some wiseacre will surely try to make it a pretext for closing the board again. You would not want to be the cause of that, would you?

[speakertoanimals]

WHERE were the rude words in that post? Unless van der waals seems a bit rude to some people........................

And rather hypocritical, given what I have been called in the past on here, seemingly without any censure on your part...............

I don't mind not being allowed to use rude words, provided the same rule is applied to EVERYONE. And if someone gives an indication as to exactly what is supposedly rude..............Is fool okay, if I can't use id**t? And why can't I type 'lies in' (valid mathematical expression), without it coming out as insights.......................

[speakertoanimals]

Back to the strong force.

It really is a bit of a thing -- in em we just had charge, positive or negative, but in the color force, we have three color charges, and their 'negatives'. So we have red, green and blue for quarks, and anti-red, anti-green and anti-blue for anti-quarks! Adding R, G and B gives a colorless (white) composite, as does adding the three anti-colors.......................

Gluons can have two color combinations. So a red quark can emit a red-anti-blue gluon (and become a blue quark in the process). A blue quark can then absorb the red-anti-blue gluon and become a red quark!

All of which makes for pretty diagrams (as long as you can remember what pen you need to use for anti-blue), but suggests that using color (and flavor) means that particle physicists were suffering from a serious lack or originality when they came to quarks and the color force. And even the word quark itself................

Outrageous puns abound in particle physics I'm afraid, as the use of 'barn' (as in a barn door, the one you couldn't hit) as a unit for cross-section in colliisons....................

[principled]

Thanks for the post. Never heard of color forces before...my ignorance. Can you explain a bit more about this "island of stability", which I think the scientists say will start around atomic number 122(?).

For example, I assume that 117 Ununseptium is electrically neutral, yet decays quickly, but if we were able to "fabricate" an atom at 122 (name???), which I assume is also electrically neutral, the nucleus is much more stable and the decay would be much slower.
I'm on dodgy descriptive ground here, but I think you get the gist...ie something fundamental happens as we hit around 122.
P

[speakertoanimals]

Golly, that a hard one!

First thing is the shell model.

from your chemistry, you may recall that electrons in atoms are arranged into discrete energy levels, and that those levels are further grouped into shells. The point being, each shell can only hold so many electrons.

This is what explains the patterns in the periodic table -- hydrogen, lithium and sodium have patterns 1, 2,1, and 2,8,1

Hence innermost shell can hold 2 electrons, next one out 8. Hence pattern in the elements since all these have 1 electron in the outermost shell.

Noble elements are helium (2), neon (2,8), and argon (2,8,8), which all have a FULL outer shell.

Okay, now back to nuclei. We have a similar pattern of shells, with neutrons and protons having separate shells (because they are DIFFERENT, you can tell the difference between a neutron and a proton, but not between a neutron and another neutron). The numbers here (the magic numbers, total numbers of nucleons that are particularly stable) are 2, 8, 20, 28, 50, 82, 126.

Except for protons, predicted magic number at the top end is actually 114 rather than 126............

And additional predicted magic number for neutrons is 184.

Hence predicted 'island of stability' for nuclei in that ball-park.

WHY more stable? Lifetimes? What matters when we consider a nucleus is the binding energy per nucleon -- how much energy we would have to add to pull the whole thing apart, and send each nucleon off to infinity. not because we are going to do this, but because the binding energy per nucleon for two nearby nuclei determines whether or not nuclear decay from one to the other can occur.

The shell model gives a larger binding energy per nucleon at the magic numbers of protons/neutrons. Kind of the like the fact that neon, helium etd don't do chemistry, because they have full shells, thank you very much, and don't want to share electrons with any other atom or form chemical bonds.

Same goes for these nuclei, although they will decay, because there are just two many ways they can spit out an alpha particle and move to a state of lower energy. But they only need to be a bit more stable than neighbouring stuff in order to be noticed.

[speakertoanimals]

I assume that 117 Ununseptium is electrically neutral,

No nuclei are electrically neutral, because they contain protons! Atoms are though, but the presence of electrons whizzing round outside doesn't have much to do with the stability of the nucleus itself (apart from some weird forms of radioactivity such as electron capture, where the nucleus itself chomps one of the orbiting electrons to convert a proton to a neutron!)