Heviest Atom

[RossFW]

Heaviest Atom

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Ununoctium has the highest atomic weight in the current periodic table.

Is there a theoretical limit to the atomic weight of an Atom? Could heavier elements exist?

And is it possible for a heavier atim than Uranium to exist in a stable state?

[Lukraak_Sisser]

I'm sure there is some theoretical limit to the atomic weight, though I don't remember enough of physics to give an awnser to that.

As for your second question, in theory, yes but they are so far not yet reached
Island of stability

[sol invictus]

And is it possible for a heavier atim than Uranium to exist in a stable state?

Uranium isn't stable. Its most stable isotope is 238, but even that decays after a few billion years.

In fact no element heavier than iron is stable.

http://upload.wikimedia.org/wikipedia/commons/5/53/Binding_energy_curve_-_common_isotopes.svg

[edd]

I was just checking a few things related to this, and I've come across references to Ni-62 being more stable than Fe-56.

[sol invictus]

I was just checking a few things related to this, and I've come across references to Ni-62 being more stable than Fe-56.

IIRC Ni-62 has the largest binding energy per nucleon. But it also has a larger neutron/proton ratio, and neutrons are heavier and can decay to protons.

So let's see, take 26 protons and 30 neutrons from Ni-62 and make Fe-56. That costs you a little, since Fe has slightly larger binding energy/nucleon. You've got 2 p and 4 n left. From that... from that we could make Li-6 and gain one proton-neutron mass difference. I'm not sure if that works, I'd have to check the numbers.

Or given two Ni-62s we could make 3 He-4s.

[edd]

Makes sense. I'm just surprised to even learn that about nickel :-)

[Tubbythin]

In fact no element heavier than iron is stable.


How are you defining "stable"?

[sol invictus]

How are you defining "stable"?

One definition is the following: take a single atom in an otherwise completely empty universe, and wait an infinite amount of time. If the atom is still there it's stable.

Another is this: take many atoms of the substance in question in an otherwise completely empty universe, and wait an infinite amount of time. If the atoms are still there they are stable.

Interestingly, if we assume protons are stable I think Ni-62 is stable according to the first but not the second definition. On the other hand protons cannot be completely stable in the standard model coupled to gravity, so really no atoms are stable (they can all decay into photons and some neutrinos).

[Cuddles]

Is there a theoretical limit to the atomic weight of an Atom?

No. In fact, things like neutron stars can be thought of as essentially just very large atoms.

Could heavier elements exist?

Yes.

And is it possible for a heavier atim than Uranium to exist in a stable state?

Possibly. The stability of an atom depends on the filling of energy shells inside the nucleus. This is similar to the situation with electron shells in atoms, where a full shell such as in the noble gasses results in a more stable and less reactive atom than a partially filled one. It is predicted that there should be an "island of stability" for heavier nuclei with particular numbers of protons and neutrons, and there are likely to be further such islands at even higher numbers. I don't think it is expected that the first island that could be reached would contain any truly stable nuclei, but at least some people have speculated that higher islands would do. Of course, until we actually manage to produce such heavy nuclei we won't know for sure.
http://en.wikipedia.org/wiki/File:Island-of-Stability.png

In fact no element heavier than iron is stable.

Except for all those stable elements that are heavier than iron.
http://en.wikipedia.org/wiki/File:Periodic_Table_Radioactivity.svg

[sol invictus]

Except for all those stable elements that are heavier than iron.
http://en.wikipedia.org/wiki/File:Periodic_Table_Radioactivity.svg

Except they're not. They may be meta-stable with a long lifetime, but they are not stable.

Protons are almost certainly not stable either, even though the experimental constraint on their half-life is much longer than the age of the universe (bonus points if you can figure out how that's possible).

[Tubbythin]

Protons are almost certainly not stable either, even though the experimental constraint on their half-life is much longer than the age of the universe (bonus points if you can figure out how that's possible).

How what's possible?

Proton decay under the Georgi-Glashow model was meant to be something like:

p -> pi0 + e+

via virtual X or Y particles (of which I know almost nothing about other than they were meant to be very heavy). But the GG model under-estimated the lifetime of the proton anyway.

Or do you mean how to measure something that's half-life is longer than the age of the Universe? Look at a lot of them. Ie 1032 protons for 1 year rather than 1 proton for 1032 years.

[sol invictus]

How what's possible?
<snip>
Or do you mean how to measure something that's half-life is longer than the age of the Universe? Look at a lot of them. Ie 1032 protons for 1 year rather than 1 proton for 1032 years.

Right, that. Bonus points for you.

In quantum mechanics anything that isn't forbidden happens. Therefore the proton decays unless there is a conservation law that forbids it to. Conservation of energy doesn't do it, obviously (it just decays into stuff with the same total energy). Same for momentum and angular momentum. Charge doesn't do it, as there are lighter positively charged particles (e.g. the positron).

The only conservation law that might forbid it is baryon number conservation. But baryon number is anomalous in the standard model (i.e. it is not a real symmetry), so that can't do it either.

Baryon number minus lepton number might be a real symmetry (it is non-anomalous), but does not forbid proton->positron + 2 photons, for example.

[Tubbythin]

One definition is the following: take a single atom in an otherwise completely empty universe, and wait an infinite amount of time. If the atom is still there it's stable.

Another is this: take many atoms of the substance in question in an otherwise completely empty universe, and wait an infinite amount of time. If the atoms are still there they are stable.
Nice practical definition there.:)


Interestingly, if we assume protons are stable I think Ni-62 is stable according to the first but not the second definition.
:confused: Please do explain.

[sol invictus]

Nice practical definition there.:)


Tough to verify experimentally, yes. Theoretically though there's no problem.

:confused: Please do explain.

Ni-62 has a larger binding energy per nucleon, but only slightly. But because it also has a larger neutron/proton ratio, its total energy/nucleon (total energy here is mass minus binding energy) is larger.

Therefore even if we pretend the proton is stable, Ni-62 could decay into Fe-56 + stuff. The problem is that the leftovers of that single atom would have to form Li-6, which has a much smaller binding energy/nucleon, and I think (although I didn't check the numbers) that that would add up to larger energy. However if several Ni-62 atoms decayed the end products can combine into things like Fe-56 + a small remainder, and with enough Ni-62 that will always be energetically allowed (and therefore it will happen).

I didn't think very hard about that and might be wrong, so take that with a grain of salt.

[Tubbythin]

Possibly. The stability of an atom depends on the filling of energy shells inside the nucleus. This is similar to the situation with electron shells in atoms, where a full shell such as in the noble gasses results in a more stable and less reactive atom than a partially filled one.
This only rather loosely true. To take nickel as an example... Three isotopes of nickel are known about that have closed neutron and proton shells (A = 48, 56 and 78). Of these three, none are stable. But nickel-58, with just two neutrons outside the closed shell is stable.


It is predicted that there should be an "island of stability" for heavier nuclei with particular numbers of protons and neutrons, and there are likely to be further such islands at even higher numbers. I don't think it is expected that the first island that could be reached would contain any truly stable nuclei, but at least some people have speculated that higher islands would do.
Really?

[Tubbythin]

Ni-62 has a larger binding energy per nucleon, but only slightly. But because it also has a larger neutron/proton ratio, its total energy/nucleon (total energy here is mass minus binding energy) is larger.

Therefore even if we pretend the proton is stable, Ni-62 could decay into Fe-56 + stuff. The problem is that the leftovers of that single atom would have to form Li-6, which has a much smaller binding energy/nucleon, and I think (although I didn't check the numbers) that that would add up to larger energy.
Yup. the Fe+Li to Ni mass ratio would be 1.000350631.


However if several Ni-62 atoms decayed the end products can combine into things like Fe-56 + a small remainder, and with enough Ni-62 that will always be energetically allowed (and therefore it will happen).

I didn't think very hard about that and might be wrong, so take that with a grain of salt.
It would certainly make an interesting Feynman diagram.