How can the properties of the same element differ

Discussion in 'Physics & Math' started by neelakash, Dec 22, 2009.

  1. neelakash Registered Senior Member

    Messages:
    491
    Hi friends,
    Can you help me to seeing the meaning of the following pat from Gottfried's book on qm?to illustrate the failure of classical physics before the invention of quantum physics,he says---

    "classical physics cannot explain why the properties of a sample of an element are identical to that of any other sample of the same element irrespective of their prior chemical or physical history-whether one sample has been extracted from one compound and another from a different compound by totally different methods...

    I cannot see what is the point of the authir.Dalton told in 1808 that the same element has identical atoms...so...if we are sure we have the same element from two or more different sources it should be obvious they would have the ientical proprties...Isn't it?
     
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. Physics Monkey Snow Monkey and Physicist Registered Senior Member

    Messages:
    869
    Personally, I wouldn't spend too much worrying about precisely what "classical physics" can and can't do.

    Still, it's interesting to think about what gottfried might have meant. Here is one attempt I just made up.

    Let's suppose that Dalton did show, as you claim, that all atoms of a given element are in some sense identical. If we regard this as experimental input, we can ask for a theoretical understanding of this fact.

    Classically, you might have thought, as some Greek scholars did, that atoms were like little balls. And furthermore, these little balls could be decorated with all kinds of little widgets and spikes and grooves and so on which distinguish one element from another. For example, let's imagine oxygen has 2 widgets, 2 spikes, and 0 grooves, while nitrogen has 2 widgets, 1 spike, and 1 groove.

    All atoms of a given element are the same in so far as they have the same number of widgets, spikes, and grooves. However, we might imagine that each feature has some internal degrees of freedom. For example, maybe widgets can be "on" or "off", or maybe spikes can rotate around some axis. This would mean that oxygen has internal structure which could potentially depend on its history. For example, maybe oxygen atoms coming from an oven have their widgets randomly distributed between "on" and "off", but oxygen atoms in water always have their widgets in the "off" position. If the state of a widget doesn't change in time as an atom sits by itself, then each atom would have some "long term" memory of its previous experiences.

    Now of course this is all pure fantasy, but the point of quantum mechanics is that it does permit us to understand quite precisely what the internal structure of atoms is like. In particular, it permits us to understood why atoms can be different and yet retain no memory of their past. Simplifying a bit, quantum mechanics tells us that no matter how you prepare an atom, if you wait long enough the atom while decay to its ground state. This is roughly the statement that atoms have no "long term" memory, and quantum mechanics predicts this quite clearly.

    For whatever it's worth, if we did have a workable classical theory of atoms, then maybe this theory would also predict that atoms have no memory. This is partially why I'm not sure gottfried's comment should be taken too seriously, but in any event, this classical theory doesn't seem to exist.
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. rpenner Fully Wired Valued Senior Member

    Messages:
    4,833
    Classical physics can't model electrons in discrete shells or their chemical properties or predict the quantization of charge which results in an electron having equal and opposite charge to a proton and every electron being the same. Since chemistry is all about electrons and their shells, classical physics is largely unilluminating as to the nature of the elements and the patterns seen in their arrangement in the periodic table.
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. scifes In withdrawal. Valued Senior Member

    Messages:
    2,573
    don't know much of quantum theory stuff, but one of the easiest ways for the same elements to have different properties, is to be allotropic, meaning it's crystal structure differs, usually because of high temperatures.

    to help you visualize it, think of iron, normally its atoms would be arranged in a repeated pattern, that pattern is a cube, and there's one atom in every corner of the cube, plus one in the middle of the cube. which is called a base centered cubic structure, bcc for short.

    at certain high temperatures, the mentioned crystal structure changes from bcc to another crystal structure, called fcc, short for face centered cubic, which is simply bcc but add one atom in the middle of every face of the cube...

    meaning; every two bcc cubes will merge into one slightly bigger fcc cube.

    and that my friend, changes the properties a lot.

    i'm not sure how exactly in iron is that, but hey, diamond and carbon are the same element, different crystal structure.

    this may not relate to the quantum stuff much, but i think it answers your thread title.
     
  8. neelakash Registered Senior Member

    Messages:
    491
    @physicsmonkey,I see what you mean...I will think about it and will write later...

    @ scifes,you are absolutely correct...but here we are talking about the same allotropic form,I presume...
     
  9. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,730
    I don't really know if this is relevant to the OP, but a physics pal one explained it something like this:

    Suppose I place 2 absolutely identical objects on a table in front of you and invite you to close your eyes for a coupla seconds. When you open them, I ask you to say whether I have switched their positions or not, assuming this is not obvious from the fact, say, they are not configured exactly as before .

    Of course you cannot say, but I can, because I know their "history" id est whether I switched them or not, and therefore I know what it means for them to be "different" objects in some sense.

    Classical physics assumes (it seems) that we ALL have our eyes open all the time.

    Quantum physics makes no such assumption. Specifically, if one assumes that the "history" of our objects refers to their time-dependent evolution in space, then Heisenberg tells us this "history" can never be known, for this would require the possibility of simultaneous measurement, to some arbitrary precision, of position and time-dependent displacement.

    Quantum theory thus asserts that identical objects are actually the SAME object.

    But there again, I am not, never pretended to be (nor ever wished to be ) a physicist; I learn my physics in bars, mainly.......
     
  10. BobG Registered Senior Member

    Messages:
    162
    I don't think Quantum Theory makes such a strong statement (although I believe that's an interpretation which some have read into it). It does however encompass the concept of particles being identical in the sense that it is fundamentally impossible to distinguish between them. If this is the case then atoms of the same element are going to have the same properties regardless of their histories.
     
  11. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,730
    Well, you can think, or (dis)believe whatever you want, of course, but unless and until you can make a coherent argument for your (dis)beliefs, then I for one do not feel compelled to accept them

    So please distinguish this assertion from the one that I made.

    Make an argument to show that these assertions are different.

    Please.
     
  12. neelakash Registered Senior Member

    Messages:
    491
    Following the ideas of physicsmonkey,in those days,it was usual thought that materials will have some memory.Chemistry was a superficial subject (people did not know the electronic configurations of the atoms etc],it must have seemed enigmatic why different samples of the same element (which also have different history) would produce identical properties (even though Dalton told the atoms are identical,that does not mean the properties of the macroscopic bulk of samples will be the same).For example,we may have extracted iron(whatever it looked like) from FeSO4 and also from a mine.They are the same element as they report identical properties-even coming of two orthogonal background---as if they do not have memory.I think Gottfried touched upon this idea.
     
  13. kevinalm Registered Senior Member

    Messages:
    993
    A little nitpicking...
    diamond/graphite/amorphous carbon
    red/white/yellow phosphorus
    plutonium has several phases iirc
    I'm sure there are others.
     
  14. BobG Registered Senior Member

    Messages:
    162
    Well I interpreted "identical objects are actually the SAME object" as in there is only one of each fundamental particle in the universe that somehow appears to be everywhere at once. Some people for example have for example proposed that there is only one electron in the universe. See

    http://en.wikipedia.org/wiki/One-electron_universe

    This is clearly an interpretation that follows from indistinguishability but does not necessarily follow from it.

    However re-reading your sentence, you may not have been referring to this.
     
  15. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,730
    Bob, in this season of good will, I apologize for my somewhat caustic remarks, truly.
     
  16. neelakash Registered Senior Member

    Messages:
    491
    The idea is of a classical atom.Since, the atoms were to have internal structure (evident from excitation spectrum experiments),people thought of them to have some internal structure.Samples of the same element from different sources were thought to contain the same atom with different internal structures corresponding to parent history.

    [input from physicsmonkey] Classically, we might have thought, as some Greek scholars did, that atoms were like little balls. And furthermore, these little balls could be decorated with all kinds of little widgets and spikes and grooves and so on which distinguish one element from another. For example, let's imagine oxygen has 2 widgets, 2 spikes, and 0 grooves, while nitrogen has 2 widgets, 1 spike, and 1 groove.

    All atoms of a given element are the same in so far as they have the same number of widgets, spikes, and grooves. However, we might imagine that each feature has some internal degrees of freedom. For example, maybe widgets can be "on" or "off", or maybe spikes can rotate around some axis. This would mean that oxygen has internal structure which could potentially depend on its history. For example, maybe oxygen atoms coming from an oven have their widgets randomly distributed between "on" and "off", but oxygen atoms in water always have their widgets in the "off" position. If the state of a widget doesn't change in time as an atom sits by itself, then each atom would have some "long term" memory of its previous experiences.

    Since,in those days,the idea of chemistry was not well developed,people used to think that the sample I have in hand contains that decorated form of the particular atom corresponding to the history of the parent compound.Testing the compound was thought to give properties manifesting the history of the parent location.It was a surprise of the classical theory of the atom that all these properties turned out to be the same.
     

Share This Page