What makes an atom unstable?

marlon

Like I already said : this is because of the numbers of nucleons occupying a certain energylevel. The things I wrote in the electron-example are basic results of QM. I mean, these results are proven and that is it. Ofcourse You can keep on asking why are these results true ??? However this is not how physics is done. Physics tries to describe nature, it does not tell nature how it has to work nor does it EXPLAIN why nature behaves in a certain manner...


regards
marlon

humanino

The stability of the proton would have been proven ? How did you achieve that ?

marlon

What the hell is this all about ???

And what do you mean by the stability of the proton. Are you referring to the asymptotic freedom of the strong force ???

regards
marlon

NanakiXIII

Thanks for all your replies, but so basically, they don't know?

humanino

Yes we do know.
Did you check my link to another thread ?
In this the best known model for nucleon cohesion : Skyrme, is derived from quark/gluon level This paper is to be published soon in PRL. Is it not knowing ?

NanakiXIII

I only looked for the answer in posts after my most recent question.

From this I derived that they didn't know.


I went back to the links you posted (that link you posted just now gets me an access denied error) and found something about closed shell configuration. I'm not sure I understand - if a shell holds a certain amount of nucleons (is full?), it's most stable?

I'm not very familiar with these shells, but they enclose eachother? One thing I don't understand then, though, if I look at this image:



The red part. It looks to me like each shell needs its own amount to be full, which are equal to the magic numbers. So if the first two shells are full, it should be the most stable (for a two-shell atom), right? But Z and N wouldn't specifically be equal to the magic numbers.

marlon

Indeed we know this very well. Look, the "total" potential energy (i mean the energy off all interactions that take place inside an atom because of all kinds of interactions that are very well understood and backed up by many many experiments) of an atom with only half of the possible "nucleon-positions" filled up is lower then an atom with let's say 6 positions filled up (let's say there are maximum 10 positions to fill up in order to have a full level just like in the Aufbau-principle). A lower potential energy-state corresponds to a more stable state.

You gotta compare this with the potential energy between the electron and the proton in Hydrogen. There you will also find a minimum for the potential energy of this two-body-system just when the two particles have a specific distance between them. It is at this distance that the electron orbits the proton that constitutes the nucleus of hydrogen and no other distance...


regards
marlon

NanakiXIII

Then wouldn't 0 be the most stable state and not the magical numbers? Maybe I'm confusing things now.

humanino

You are indeed confusing : link in post 8, Fe56 is the most stable configuration, for which the nucleons are the most strongly bounded together. You can also see in the "Fission and fusion can yield energy" graph, that even for light nuclei, the binding energy raises very fastly. The local maximums can also be understood in "Binding Energy for the Last Neutron as Evidence of Shell Structure" in the link of post 7.

When one shell is closed, it is not easy to take away a nucleon by striking it. It holds with his buddies together. If there is a "hole" in the structure, it gets weaker. It is the same in football I guess.

reilly

In non-relativistic QM, the process of tunneling through a potential barrier gives a very good model for radioactive decay, like alpha decay. But, in the relativistic domain, this approach was not fruitful. Rather, Fermi had the profound insight that the best he could do was to describe the beta-decay/weak interaction as a point interaction of quantum fields, suggested by the E&M interaction, -- forget the bound states, and complicated dynamics. He kept it simple, and for almost 60 years his formulation of waek interactions was IT, until suplanted by the Standard Model.


Quantum field theory is a theory of transformations: electrons change into electrons plus photons, leptons change into leptons and w-vector bosons; neutrons change into protons, electrons, and anti-neutrinos, and so on. These ideas are incorporated into field theory by means of local point interactions of quantum fields. In turn, appropriate mathematical manipulations yield interactions in terms of creation and destruction operators, as in:

(create photon)*(create electron)*(destroy electron)

or, as Fermi postulated for beta-decay

(create electron)*(create antineutrino)*(create proton)*(destroy neutron)

The process of interaction is laid out as a step-by-step process -- the picture of such a process is a Feynman diagram. The fermi interaction is represented by 4 lines radiating from a point

Here's an area where some fundamentals of QM are directly driven empirically. That is: we know atoms, nucleii and particles decay. But more generally. transformations can be seen in scattering experiments, say like photoproduction of charged pi mesons. So, in one way or another, theory must accomodate transformations among particles.

The structure of QFT interactions guarantees radioactive decays, pair production, radiation, inelastic scattering and so forth. That particles can transform is a basic assumption of QFT. Thus, we can't do much about the core why. But we can certainly describe and predict many phenomena given the QFT interactions. As always, there are three major conditions:

The mass of the decay products must be less than the mass of the parent
(Energy must be conserved along with momentum)

Angular monentum must be conserved

Selection Rules must be obeyed
(A positron cannot absorb a photon and turn into a proton;charge must
be conserved,....)

So, QFT does a great job of describing decays, but, ultimately, we really don't know quite why the decays actually happen. But then, we don't know why electric charge comes in two non-zero flavors, nor why our everyday space is 3D, nor ... Great mysteries.

Regards,
Reilly Atkinson

vlamir

Thanks Reilly!
It is the excellent analysis of the problem!

NanakiXIII

Thanks you all for trying to explain this to me, but I think I'm in over my head. I understand that a certain configuration is most stable, and if an atom isn't stable, has a not very great configuration, it decays. The rest is just a big blur of information. If Z or N or both are equal to one of the magic numbers, it's very stable, if I understand correctly. But beyond that, I'm just lost. So I think I'm just going to leave this. Maybe I'll be able to understand more in time.

But, thanks a lot anyway, I really appreciate all your help.

Chronos

Dang humanino, you give big answers to small questions. Theoretical physicists are a pain in the accelerator. Simple version [?]. Mother nature is lazy. Even a lowly atom will seek a lower energy level given the opportunity. An atom will spit out an alpha particle, beta particle or even a lousy gamma ray given the opportunity. Radioactivity is a consequence of the weak nuclear force: which is just another way of saying mother nature is lazy.

geometer

Nanaki - I've read some of the replys here and they confuse me too. First, you need to be familiar with what appears to be a basic principle of nature, and that is that nature likes to minimize potential energy (I realize guys this is oversimplified, but this is a basic explanation). Now, to radioactivity. As I am sure you're aware, the nucleus is made up of protons and neutrons. These neutrons and protons fill energy shells in the nucleus just like electrons fill energy shells around the nucleus. If some of these shells have too few or too many nucleons, the atom is not in it's most desirable energy state and acts to achieve that.

For too many neutrons, the atom converts a neutron into a proton and emits a negative particle known as a Beta-minus particle (it is essentially an electron)

For too few neutrons (in other words, too many protons), the atom converts a proton into a neutron and emits a positive particle known as a Beta-Plus particle (this is essentially an anti-electron)

For atoms that are way too big like uranium, the nucleus just spits out a chunk of itself - a particle consisting of 2 protons and 2 neutrons known as an alpha-particle.

In addition to spitting out these particles, these unstable nuclei usually emit a high frequency photon known as a gamma particle.

There are other more exotic modes of radioactive decay but these are by far the most common types encountered.

geometer

In your example of H-3, more commonly known as tritium, this is a hydrogen nucleus that contains one proton and two neutrons. A normal hydrogen nucleus is just one proton so this nucleus contains too many neutrons and it decays by Beta-Minus decay, converting one of those neutrons into a proton and emitting a Beta-Minus particle as we talked about above.

marlon

Just as a little addendum to the explanations of geometer i would like to stress the fact that the emitted electron does NOT come "out" of the nucleus. Basically, following the rules of QFT it is created out of the so called fysical vacuum (out of nothing, if you wish). The energy needed for this creation come from the decaying mother-nucleus itself via the well known formula E=mc²...

marlon

vlamir

Certainly this is very "smooth" pedantic explanation of stability and instability of isotopes, which offer geometer and marlon.
I assume, that sub-forum Theory Development should pursue other purpose – to offer an explanation for those phenomena which have no a satisfactory explanation in handbooks.
Let's look at stable and long-living isotopes of those elements, which are located before noble gases in the Periodic table:
1H (1p) – stable;
2H (1p+1n) – stable;
3H (1p+2n) – unstable: emanation of electron (beta–);

18F (9p+9n) – unstable: emanation of proton (beta+); electron capture;
19F (9p+10n) – stable;

35Cl (17p+18n) – stable;
36Cl (17p+19n) – unstable: beta+; beta–; electron capture;
37Cl (17p+20n) – stable;

77Br (35p+42n) – unstable: electron capture; beta+; gamma;
79Br (35p+44n) – stable;
81Br (35p+46n) – stable;
82Br (35p+47n) – unstable: beta+; gamma;

123I (53p+70n) – unstable: electron capture; gamma;
125I (53p+72n) – unstable: electron capture; gamma;
127I (53p+74n) – stable;
129I (53p+76n) – unstable: beta–; gamma;
131I (53p+78n) – unstable: beta–; gamma;

210At (85p+125n) – unstable: electron capture; alpha;
211At (85p+126n) – unstable: electron capture; alpha;

As you see, even this limited list of isotopes does not submit to the common rule. Especially it concerns to other isotopes (chlorine has 13 isotopes, bromine – 28, iodine – 37).
I think, there are big opportunities to explain stability and instability of isotopes due to features of a design of atoms (due to features of a geometry), but not amount of protons and neutrons in their center.