[tweener (OP)]

In terms of atomic number, what element is the largest naturally occuring atom?  I know there are several man-made elements that are larger but unstable.  Why are they unstable?  Why is there an upper limit on the size (in nature anyway)?

----
John - The Eternal Pessimist.

[chris]

Great question, I only wish I knew the answer with certainty. It is almost certainly a function of nuclear forces.

The abundant natural elements on Earth are the vestiges of long dead stars which powered themselves by fusing smaller nuclei to make larger ones. But the process only yields energy as far as iron (Fe). Thereafter it is no longer energetically favourable (via the nucleosynthesis mechanism) to make larger nuclei. Hence the abundance of iron, and the relative scarcity of the largest elements.

But in terms of producing larger and larger elements de-novo, as the nucleus become larger and larger I would speculate that it becomes more and more difficult with increasing atomic number to reach sequential proton / neutron configurations that are sufficiently stable to make the element persist for a measurable length of time.

In other words, the intermediates that you would need to build upon (by adding more neutrons / protons) to make anything larger don't exist for long enough to make that possible, and the amount of energy that you would need to supply to add more particles to the nucleus becomes preclusively enormous.

This is pure conjecture on my part. I'd be extremely grateful for the input of a nuclear physicist !

Meanwhile, Prof. Brian Fulton, from York University, put together a nice page about nuclear astrophysics (including nucleosynthesis) for his appearance on The Naked Scientists. Here's a link to his page :

http://www.thenakedscientists.com/astrophysics/

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[cuso4]

Most atoms have more neutrons than protons, this is to increase the nuclear force without increasing the electrostatics repulsion (produced between protons) and hence making the nuclei more stable.

Very large nuclei are unstable because strong nuclear force that holds the nucleons together has a very short range so cannot reach every nucleon. The following shows how nuclei become more stable:


Also have a look at the graph for binding energy per nucleon:


Fe-56 is indeed the most stable nuclei.

Source: http://europa.eu.int/comm/research/energy/fi/fi_bs_np_en.html

Angel

[gsmollin]

quote:Originally posted by tweener

In terms of atomic number, what element is the largest naturally occuring atom?  I know there are several man-made elements that are larger but unstable.  Why are they unstable?  Why is there an upper limit on the size (in nature anyway)?

----
John - The Eternal Pessimist.

I'm not sure you got your answer, although you did get a lot of information. The heaviest naturally occuring atom is uranium. Uranium is radioactive, and decays into lead, which is the heaviest stable element.

Cuso4 answered your second question.

[Ylide]

Actually, bismuth is the heaviest stable (i.e. non radioactive) element.  Lead is more dense, but not heavier in terms of atomic weight.  

Also, neptunium and plutonium, which are heavier than uranium, are present in trace amounts in uranium ores, but are more commonly produced synthetically.  




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[tweener (OP)]

Thanks!  This is great information.  I was reading some and saw a reference to Bohr's "liquid drop" model of the nucleus.  All I got from that was that the strong force can't overcome the repulsion as the nucleus gets large (as Angel shows).  But there were no details of where or why the cutoff occurs.

Chris brings up an interesting point about stellar nucleosynthesis stopping when the energy required gets large.  I wonder if there is also something about the high energy environment that helps stop it also.  In a collapsing star, there would be a lot of neutrons flying around causing fission at the same time the temperature and compression are trying to fuse nuclei together.  Maybe U is the tradeoff.

The trace amounts of neptumium and plutonium may come from sporadic decay of Uranium after the star has collapsed.  When the U emits a neutron, it may hit another U and make a Pu nucleus.

I really don't know much about this, but it's interesting.

----
John - The Eternal Pessimist.

[Dan B]

I think technically the largest atom will be a neutron star []

[qpan]

Well, suppose that would depend on your definition of an atom... although that does make black hole the heaviest element of them all...

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

[chris]

quote:Originally posted by Dan B

I think technically the largest atom will be a neutron star []

Clever ! That would make the largest atom about 12km across then, although I'm not sure that a neutron star entirely fits the criteria of an atom in our present sense of the word.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Dan B]

[] yeah, a neutron star is more dense than the atomic nucleus.

[cuso4]

quote:Originally posted by Dan B

[] yeah, a neutron star is more dense than the atomic nucleus.

I don't think it's 'more' dense than the atomic nucleus. A neutron star is 'as' dense as the atomic nucleus.

Angel

[Dan B]

yeah, sorry, I was getting my degeneracies mixed up. [:I] Currently bashing my head with a large book on nuclear physics.

[tweener (OP)]

Don't you do that all the time Dan? []




----
John - The Eternal Pessimist.

[Dan B]

nah, usually its a book on stellar dynamics. [yawn]

[Ylide]

To meet the definition of an atom, it would need to have one distinct set of electron orbits for the entire particle.  (Maybe particle isn't the right word for something 12km across)

In the case of a nuetron star, I believe all the electrons are sucked into the nuclei of each atom that originally made up the star, causing the protons to become neutrons as well.  A big wad of neutrons does not make an element, since it really doesn't have an atomic number or any way of classifiying it along the periodic table.

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[Dan B]

quote:Originally posted by cannabinoid
I believe all the electrons are sucked into the nuclei of each atom that originally made up the star, causing the protons to become neutrons as well.

yeah, this is degeneracy. Its neutron degeneracy pressure that stops the neutron star collapsing further.

[qpan]

quote:Originally posted by cannabinoid

To meet the definition of an atom, it would need to have one distinct set of electron orbits for the entire particle.  (Maybe particle isn't the right word for something 12km across)

In the case of a nuetron star, I believe all the electrons are sucked into the nuclei of each atom that originally made up the star, causing the protons to become neutrons as well.  A big wad of neutrons does not make an element, since it really doesn't have an atomic number or any way of classifiying it along the periodic table.

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A neutron star may not be an element, but (i think) you can't discount it as an atom on the basis it has no electrons- would you say that a H+ ion is not an atom because it has no electrons?
Also, there are strictly speaking, no such things as electron orbitals, only areas of high probability of finding electrons. Therefore, theoretically speaking (on a quantum mechanical basis) you could not discount the fact that an atom has electrons "orbiting" it even though there are on no electrons in the close vicinity (as the electons have an infinitely small possibility of being an infinitely far distance away).

I know i'm being awfully pedantic, but i'm just questioning the true definition of an atom.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

[Ylide]

gpan,

A neutron star ostensibly doesn't have protons either, as they've captured the electrons of each original atom to become neutrons.  I can't think of anything fitting the definition of an atom that contains neutrons and only neutrons.



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[OmnipotentOne]

Hmm all of this is really interesting!  Except i cant understand the graph in the leist.


OO

[Ylide]

The graph is a plot of the number of neutrons vs the number of protons in a given atom.  There is a certain ratio of neutrons to protons that is naturally stable, shown by the white region in the middle of the graph.  The graph isn't highly resolved so it's hard to give specific examples, but if you look right around 6 protons (carbon) and note that carbon-12, the most stable isotope, has 6 an AM of 12, you'll see that 6 protons graphed vs 6 neutrons falls in the white region of the graph.  Carbon-13, which has 7 neutrons, is outside the valley of stability and hence is radioactive.  That help?



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