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Thread: About limit for atomic number

  1. #1
    Registered Senior Member Eagle9's Avatar
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    About limit for atomic number

    As well-known the atomic number uniquely identifies a chemical element, for instance Hydrogenís atomic number us equal to one, Uraniumís is equal to 92 and etc. I wonder, is there any limit for atomic numbers and hence the number of chemical elements? Letís say, can there exist in nature the element with atomic number of 156 or 270? I know that creating them synthetically would be greatest challenge, but now I would like to know if they generally can exist or not

  2. #2
    Theoretically you could make any element with any atomic number but it will be unstable and will eventually decay into something stable that we already know about. This picture shows the stable elements - as you can see, the largest stable number of protons is Bismuth, having 83.


  3. #3
    They can exist, but as the number of protons increases, the half-life of the substance becomes progressively shorter. You'll eventually reach a point at which the half-life is so brief that you won't have any practical way to observe the atoms, much less test them.

  4. #4
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    "Bismuth has classically been considered to be the heaviest naturally-occurring stable element. Recently, however, it has been found to be very slightly radioactive: its only naturally occurring isotope bismuth-209 decays via alpha decay into thallium-205 with a half-life of more than a billion times the estimated age of the universe."

    Above is from Wikipedia article.

  5. #5
    Quote Originally Posted by mathman View Post
    "Bismuth has classically been considered to be the heaviest naturally-occurring stable element. Recently, however, it has been found to be very slightly radioactive: its only naturally occurring isotope bismuth-209 decays via alpha decay into thallium-205 with a half-life of more than a billion times the estimated age of the universe."

    Above is from Wikipedia article.
    When you're talking about those sorts of timescales I bet there's a bunch of other "stable" nuclides that turn out to be unstable. In field theory, there aren't really any stable particles at all. A real particle is just a virtual particle that is very close to being on shell for a while.

  6. #6
    Quote Originally Posted by Eagle9 View Post
    As well-known the atomic number uniquely identifies a chemical element, for instance Hydrogenís atomic number us equal to one, Uraniumís is equal to 92 and etc. I wonder, is there any limit for atomic numbers and hence the number of chemical elements? Letís say, can there exist in nature the element with atomic number of 156 or 270? I know that creating them synthetically would be greatest challenge, but now I would like to know if they generally can exist or not

    The protons in the nucleus are kept together by the strong force. On short distances , the strong force can overpower the enormous electromagnetic force that makes protons repel each other.
    As you start "packing" more protons and neutrons in the nucleus, the distance needed to be covered by the strong force increases and the electromagnetic force starts overpowering the strong force. As such, the nucleus created by packing more and more nucleons together is no longer stable. This is why we cannot synthesize infinitely heavy elements.

  7. #7
    That's actually the residual strong force, Tach. See http://en.wikipedia.org/wiki/Nuclear_force. The strong force keeps a proton together.

  8. #8
    Valued Senior Member
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    Quote Originally Posted by prometheus View Post
    When you're talking about those sorts of timescales I bet there's a bunch of other "stable" nuclides that turn out to be unstable. In field theory, there aren't really any stable particles at all. A real particle is just a virtual particle that is very close to being on shell for a while.
    There is no evidence for your assertion, so I'll continue to assume all known stable isotopes are truly stable.

  9. #9
    Quote Originally Posted by mathman View Post
    There is no evidence for your assertion, so I'll continue to assume all known stable isotopes are truly stable.
    Indeed. My limited understanding of nuclear physics is that it's quite ad hoc and in fact quite poorly undertood. A lot more so than, for example, atomic physics which is more or less tied up by the Schrodinger equation.

  10. #10
    When I look at isotopes I tend to think of the nuclear force as "neutron linkage".

  11. #11
    Dr. of Physics, Prof. of Love BenTheMan's Avatar
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    Quote Originally Posted by Farsight View Post
    When I look at isotopes I tend to think of the nuclear force as "neutron linkage".
    This doesn't make sense.

  12. #12
    All: start with helium-3 and you've got one neutron and two protons. To visualize this imagine walking down the street with a girl on each arm. Your arms are linked. In this analogy you are the neutron, hence "neutron linkage", and a menage-a-trois such as this can be stable. Think of helium-4 in terms of you and your good buddy walking down the street with your two girlfriends, all with arms linked. That's stable too. Apply the same simple logic to carbon, oxygen, iron, and other isotopes. The more protons there are the more opportunity for stable extra links. There may be patterns to this. For example thallium-180 has 81 protons and 99 neutrons, but doesn't decay to via mercury-180 to two atoms of zirconium-90. Instead we see ruthenium-100 and krypton-80. Divide the 81 protons and 99 neutrons by 9 and you see they're in a 9:11 ratio. Then split the 9 into a 5 and a 4 and scale back up to 100 and 80, and note that flourine has an atomic number of 9. Hence thallium-180 might be akin to an aggregation of 9 flourine-19 atoms with 9 extra links. Interesting stuff.

  13. #13
    Dr. of Physics, Prof. of Love BenTheMan's Avatar
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    This sounds a bit like numerology, and I don't know if it's right. It may be that the patterns you're noticing have already been documented, you might try looking at the nuclear shell model.

  14. #14
    They haven't AFAIK, though that's not to say they aren't in papers somewhere that have received scant publicity. What does seem to get publicity is nuclear-shell-model "magic numbers". I have to say that sounds more like numerology than the neutron-linkage inference drawn from atomic number and atomic weight.

  15. #15
    All aboard, me Hearties! Captain Kremmen's Avatar
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    Quote Originally Posted by Farsight View Post
    All: start with helium-3 and you've got one neutron and two protons. To visualize this imagine walking down the street with a girl on each arm. Your arms are linked. In this analogy you are the neutron, hence "neutron linkage", and a menage-a-trois such as this can be stable.

    That's a bad analogy.
    Menages a trois are notoriously unstable.
    Often the neutron will love one proton more than the other, and just want the other as a friend.
    This causes jealousy between the protons, and often leads to tragic consequences.

  16. #16
    Quote Originally Posted by Tach View Post
    the distance needed to be covered by the strong force increases and the electromagnetic force starts overpowering the strong force.
    and the distance needed to be covered by the weak force decreases.

  17. #17
    Quote Originally Posted by Lady Historica View Post
    and the distance needed to be covered by the weak force decreases.
    Foul. What does the weak force have to do with internucleon forces?

  18. #18
    Quote Originally Posted by Tach View Post
    The protons in the nucleus are kept together by the strong force. On short distances , the strong force can overpower the enormous electromagnetic force that makes protons repel each other.
    As you start "packing" more protons and neutrons in the nucleus, the distance needed to be covered by the strong force increases and the electromagnetic force starts overpowering the strong force. As such, the nucleus created by packing more and more nucleons together is no longer stable. This is why we cannot synthesize infinitely heavy elements.
    The strong force can not overpower the strong force itself. It needs Energy from an outside influence (hence weak force). Not only does packing more protons into the nucleus increase its distance needed to be covered by the strong force, but decreases the distance to the weak force making it more ceceptabne to instability and the electromagnetic force that overpowers unstable elements.

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