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The maximum number of elements

  1. Mar 5, 2014 #1

    bobie

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    Reading this wiki article http://en.wikipedia.org/wiki/Atomic_electron_configuration_table
    I was surprized to see they predicted 172 elements.
    Now, since the speed of the electron of Hydrogen is vH = 2,18816 (C/137), and since speed in further elements is multiplied by z (number od protons) isn't the maximum possible of elements/protons 137 ?, in such element the speed of the ground electron would be = C.
    Is that correct?

    Thanks
     
  2. jcsd
  3. Mar 5, 2014 #2

    mfb

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    There is no well-defined "speed" of an electron in an orbital. There is a well-defined energy, but that is not limited. In terms of an expected speed, you have to consider relativistic effects for heavy atoms: the orbitals look a bit different, but again there is no upper limit for electron orbitals, and the expected speed is always below c.
    As a smaller effect: the orbitals get so small and the nuclei get so large that the lowest states have a significant fraction inside the nucleus, therefore the effective charge seen by the electron goes down a bit (relative to the total charge of the nucleus).

    Heavy elements have nuclear stability as a different issue - it is unclear if those heavy nuclei can exist.
     
  4. Mar 5, 2014 #3

    Vanadium 50

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    Which they didn't. Had you read it carefully, they are saying if there is a nucleus with Z = 172, this is what it's electron configuration would be. That's a very different statement than that there is an element with Z = 172.
     
  5. Mar 5, 2014 #4
    1.Wikipedia doesn't "predict". It provides information created by other sources.
    2.The "predictions", according to the article, are based on some calculations done in 1975. 1975 !?!
    3.Pay attention to numerous examples where 'ground state' configuration is 'anomalous'- specific examples:Cr & Lr. There are ~ 20 elements (20%) which don't follow "simplistic" rules. I didn't see (wasn't thourough) ANY anomalies above 104...if that doesn't smell fishy, you need to re-calibrate your nose.
    4.Speaking about speed (velocity) is a problem when dealing with Quantum Mechanical bound states. Physicists (carefully) use certain "Classical" equations in deriving certain results because they are consistent with the known quantum mechanics AND empirical results, and hence although it is fundamentally "wrong" to think about an electron having a speed in an orbital, it works for *some* purposes.
    5.It can be confusing to a novice to see these equations, since they imply (incorrectly) that the underlying physical parameter is "real". You should take to heart two things: using interpolated rules to arrive at extrapolated facts is very treacherous (but educational) and fools run in where angles fear to tread.
     
  6. Mar 5, 2014 #5

    bobie

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    I am not familiar with QM. Are they saying that (Bohr radius) rB is not .529*10-8 anymore? and speed is not C/137,and orbitals must not be nλ anymore? and the radius of helium must not be rB/2 and its speed is not 2C/137?
    If these principle are still valid then what's the use of predicting a configuration if, when they know that if is impossible
     
  7. Mar 5, 2014 #6

    mfb

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    I said nothing about the Bohr radius. But keep in mind this is not the physical distance of anything. The Bohr model is wrong.
    "The speed of an electron" does not exist.
    What does that even mean?
    "Its speed"? Of the atom? "Radius of an atom" is a problematic concept as well - the wavefunction has no border. There are some ways to define such a thing, but the result depends on the definition then.
    What is impossible?
     
  8. Mar 5, 2014 #7

    bobie

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    Is everything we studied in chemistry wrong?
    An atom has no radius,no dimention?
    electrons do not circle around the nucleus making a standing wave?
    the orbit of an electron must not be a(n exact) multiple of its wavelength?
    an electron does not take a certain time to make an orbit and its speed is not the inverse of the circumference, and is not α the fine structure constant the ratio of that speed to C?
    isn't the circumference of an atom H = 3.325*10^-8 cm and of He = 1.66^-8 cm anymore?

    Can you say what is the new model like or suggest an article where it is explained?
    Thanks
     
    Last edited: Mar 6, 2014
  9. Mar 6, 2014 #8

    Nugatory

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    It's not exactly a "new" model, as it's about 90 years old now. Google for "hydrogen atom wave function" or "hydrogen atom schrodinger" and you'll find some stuff, and any first-year QM textbook will have a chapter on this - it's the killer demo for QM the way that planetary motion is the killer demo for Newtonian gravity.

    And with said, yes, atoms have dimensions and a radius although they are a bit "fuzzy" at the edges. Yes, electrons do surround the nucleus, but no, they do not orbit or have a speed; they're more like a cloud around the nucleus. Because the electron does not orbit (don't be misled by the word "orbital", that's a historical accident) there's no orbit to relate the wavelength to, but the wavelength is still deeply connected to the energy levels.
     
  10. Mar 6, 2014 #9

    mfb

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    I don't know what you studied in chemistry.
    You can define a radius (actually, there are multiple definitions, with different values), but an atom is not like a billard ball.
    They make a standing wave, they do not circle around the nucleus.
    The multiple of a wavelength is a length, an orbit is a 3-dimensional wavefunction. They are completely different things.
    Anyway, "n wavelengths" has no relevance for orbits. That belongs to the Bohr model that is outdated for ~80 years now.
    Certainly not!
    As mentioned before, "the speed of an electron" is not a meaningful concept.
    What is the circumference of an atom?
    Basic quantum mechanics. Did you hear about the Schrödinger equation?
     
  11. Mar 7, 2014 #10

    bobie

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    I heard about the equation. I hope you could clarify these points:
    I deduce from your replies QM still concedes that there are electrons around the nucleus, so
    how can an electron make a standing wave if it is not moving and how can it resist electrostatic attraction if it has not a well defined speed?
    a 3-d wavefunction has a shape, what is it? is it spherical? what is the difference between the shape of an atom of H and He?
    I would appreciate very much if you could spend a few words on what is wrong with the model of electrons orbiting the nucleus.
    Thanks
     
  12. Mar 7, 2014 #11

    davenn

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    how about doing some research for yourself, here on PF we encourage people to learn to do research and then maybe ask sopecific questions on things they didnt understand

    an earlier responder commented about the "Electron Cloud Model"
    Do a google search on that and you will find dozens of worthy links that will answer your question

    the first link from UniversityToday has a prety good summary

    cheers
    Dave
     
  13. Mar 7, 2014 #12

    bobie

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    Thanks, dave, I read that and other articles, but the only stress the fact that we do not know exatly where the electron is and there are only probability ...etc.
    They say nothing about the physics laws, wich, I suppose are the same.
    As I said we know there is an electron in a H-atom, right? No matter if we cannot know where it is, we do now that it mus be going in a circle to stay there, and at a definite speed not to fly off or crash down. Are the classical parametrs still valid? if it has no speed or it is different fron C/137 are there no problems in the new model?
    They say:
    "The Bohr model begins to run into problems with heavier atoms. Other shortcomings of the model are:gives an incorrect value for the ground state orbital angular momentum, fails to explain much of the spectra of larger atoms, and the model also violates the uncertainty principle because it considers electrons to have known orbits and definite a radius."
    So it seems to work fine for the H-atom, but for L=h/2π , and what about the magnetic moment, is [itex]\mu[/itex] right or also 0?
    Can you tell me if this article is reliable or dated:http://en.wikipedia.org/wiki/Atomic_orbital
    Thanks for your help
     
    Last edited: Mar 7, 2014
  14. Mar 7, 2014 #13

    Nugatory

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    You are still talking as if the electron is like a little teeny grain of sand, with a definite position that we just happen not to know. It's not. It doesn't have any position at all until we measure it or do something else to localize it within a particular region of space - and a bound electron is already localized to the general neighborhood of the nucleus and that's as good as it gets.

    Not even there, because of the part that I bolded.

    At first glance it looks pretty good. However, you may not find it very helpful until you've had some practice solving the Schrodinger equation in simpler cases, and until you understand what position-space solutions to the SE are really telling you about the system in question.
     
  15. Mar 8, 2014 #14

    bobie

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    -You mean an electron is not a concrete body, a piece,unit of mass anymore?
    Does this apply to electrons in a TV cathodic tube or circling in vacuum around a magnetic field?
    What I do not understand that, granted the electron does not exist per se but is a cloud of probabilty, this cloud like anything in the world must have a shape and a size (if not a radius), what is that?
    Also,
    -What is the evidence that L= 0?
    -If you say that the article is OK, then 99% of what I studied in chemistry, if it is not, can you suggest an equivalent exaustive article on the QM-shoeredinger model?
    Thanks
     
    Last edited: Mar 8, 2014
  16. Mar 8, 2014 #15

    mfb

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    It is not something like a classical object.

    All electrons are the same, but in those setups quantum effects are less notable.

    Yes, this "cloud" has a well-defined shape. Size is more problematic, as the "cloud" has no border - it extends (in an idealized setup) to infinity, just with a negligible probability density.

    L as quantum number for the ground state? L=0 is the only option in a hydrogen-like atom in quantum mechanics. You can also show this if you study the interactions of such an atom with light.

    Chemistry often uses simplified models (physics as well, but that's not the topic here) - they don't have to be wrong, but sometimes they are not very accurate or can be misleading.
    In the same way, newtonian mechanics is not correct - but it gives a very good approximation for everyday problems, so no one uses general relativity to design a building.
     
  17. Mar 8, 2014 #16

    bobie

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    I thought that there exists only energy and mass, is it still considered mass, a material body?
    Is there an article where I can learn more of the new definition of (the nature) of the electron?

    Could you specify what shape?
    L as angular momentum, which I studied to be h/2π

    If I got it right , at least for Hydrogen, everything seems identical to what I knew:
    But, as you say the electron has no speed, soes the cloud indicate the positions of the electron or more entities. Also, I cannot imagine what happens in your new model when the electron was thought to fall from orbit 3 to to, emitting a photon with wavelength 656.28.
    Could you explain that, please?
     
    Last edited: Mar 8, 2014
  18. Mar 8, 2014 #17

    Nugatory

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    A Google image search for "electron orbital shapes" will find plenty of pictures. Just remember that the edges are of those shapes are fuzzy.
     
  19. Mar 8, 2014 #18

    bobie

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    Thanks, but the cloud for the ground state is just a sphere,https://www.google.it/search?q=elec...XNygOM_oLYBw&ved=0CAkQ_AUoAQ&biw=1242&bih=594
    what is the difference with the model that makes the electron circle describing a standing wave and this wave rotate in the normal direction. What is the gain in considering the electron not a material object made of mass that does not move? That really beats me
     
  20. Mar 8, 2014 #19

    Vanadium 50

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    Bobie, this thread started because you misinterpreted the Wikipedia article. In message #5, you misinterpreted the replies you got up to that point. In message #12, you misinterpreted the replies you got up to that point as well as the references you were pointed to. In message #14, you misinterpreted the replies you got up to that point. Same for message #18.

    Whatever this thread is doing, it is clearly not helping you understand.
     
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