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I Electron energy level values

  1. Jul 15, 2017 #1
    Consider hydrogen and helium. Both have electrons in energy level 1. When the electrons move from level 1 to a higher level, are the energy quantum the same for the electrons in hydrogen as in helium? If so, then how can one distinguish hydrogen from helium by observing the emitted light?

    Also, is there a table of acceptable energy levels of electrons vs the element itself? i.e. is there a table that shows the energy levels of hydrogen vs helium vs sodium for example.
     
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  3. Jul 15, 2017 #2

    vanhees71

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    It's not the same, because the two electrons in the helium are not only interacting with the electric field of the nuclei but also interact. That's why He has another spectrum than H.
     
  4. Jul 15, 2017 #3

    mfb

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    The charge of the nucleus is different, that leads to different energy levels.
    As a smaller effect, the additional electron (in neutral helium) has an influence as well.
    There are tables for the elements, sure. Many different tables depending on what exactly you are interested in.
     
  5. Jul 15, 2017 #4

    vanhees71

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    Of course, what I meant was the general pattern. The energy pattern of a hydrogen like ion go like ##Z e/r##, where ##Z## is the charge number of the nucleus, i.e., the number of protons (in the approximation of treating the nucleus as point particle). So all levels scale with ##Z## to begin with. On top comes also the interaction between the electrons.
     
  6. Jul 15, 2017 #5
    So here is a related question. Take tungsten, a light bulb filament. As the voltage in a light increases, the color changes from an orange to (almost) white. The light comes from the electrons jumping between energy states. So I guess the color change is because the proportion of light from low energy electrons to the high energy electrons changes. The frequency of the individual light lines are always the same. Is this correct?
     
  7. Jul 15, 2017 #6

    vanhees71

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    This is rather thermal radiation since you have many atoms in the tungsten filament, and you don't see well-defined spectral lines anymore but a continuous spectrum which should be quite well described by Planck's famous black-body radiation formula.
     
  8. Jul 15, 2017 #7
    I think I understand. When the filament is relatively cold, then the spectral lines that peak at red give the red color, then as the voltage is increased, the temperature rises and the spectral lines that peak in the blue start to dominate thereby giving the change of color. The frequency of the individual spectral lines do not change, only the magnitude of them.
     
  9. Jul 15, 2017 #8

    mfb

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    My post was written in parallel to yours, it was a reply to the original post.
    As r scales with 1/Z, it is probably better to call it a Z2 scaling

    @barryj: There are no spectral lines involved in a tungsten light bulb. It is a continuum.
     
  10. Jul 15, 2017 #9
    Wait, Wait, Wait. Are you saying that heating tungsten to the point that it glows, as in a light bulb, and then passing the light through a prism you will not see the spectral lines??
    Why not??
     
  11. Jul 15, 2017 #10

    mfb

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    The emission is thermal. It follows a blackbody spectrum. No lines (at least to a good approximation).
    You get something like this.
     
  12. Jul 15, 2017 #11
    So, how would I be able to seed the spectral lines of say tungsten if not by heating it up. As I recall, if you put a piece of copper wire into a flame you will see a blue or green color. Where does the color come from, is it thermal?
     
  13. Jul 15, 2017 #12

    PeterDonis

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    Spectral lines are observed when light is emitted from electron energy level transitions in single atoms or molecules (I'll say "atoms" henceforth for simplicity). This happens if the atoms themselves are not interacting with other atoms (or at least interacting very weakly); the best way to ensure that is to work with a gas of atoms (usually at very low pressure). So spectral line observations are made with, for example, gas discharge lamps--enclosed containers that hold vapor of various atoms:

    https://en.wikipedia.org/wiki/Gas-discharge_lamp#Color

    So if you filled a gas discharge lamp with tungsten vapor, you would see the spectral lines of tungsten. Or, if you put a piece of copper wire into a flame, some copper vapor is produced that emits the colored light you see.

    However, the metal filament in a light bulb is not a gas, it's a solid, and what's more, it's a metal, which means that atoms interact very strongly with neighboring atoms and there are not sharp, widely spaced energy levels for the electrons. The light you see from an incandescent light bulb with a tungsten filament is, as others have said, almost entirely thermal radiation, i.e., incoherent radiation with a black-body spectrum arising from the thermal vibrations of the atoms, not from electron energy level transitions. What few electron energy level transitions there are are between very narrowly spaced energy levels determined by the larger scale (many atom) structure of the metal, not by the structure of individual atoms. So you won't see anything like the spectral lines emitted by single tungsten atoms in a vapor.
     
  14. Jul 15, 2017 #13
    I appreciate this explanation but with each explanation, I tend to have father questions. If copper vaporizes in a flame then it seems that some of the tungsten would also vaporize at a high voltage. In fact, if the voltage is high enough it would melt right?

    So when we look at the spectrum from stars to identify their composition, why is this spectral and not thermal?

    Is the sun spectral or thermal?
     
  15. Jul 15, 2017 #14

    mfb

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    If the voltage is too high the tungsten filament melts and breaks. That means the light bulb stops conducting electricity, everything cools down and you have a broken light bulb.

    Tungsten is chosen because it has such a high melting and boiling point (the highest of all pure elements): Vaporization is negligible.
    It is mainly thermal from the plasma in the photosphere where the light is emitted. Above that you have the colder chromosphere, where you have atoms absorbing specific wavelengths.
    The result is a continuous spectrum where a few spectral lines are missing.

    320px-Solar_spectrum_en.svg.png

    From here. Yellow is the spectrum above the atmosphere.

    This is different from mercury lamps, for example, where you only have the spectral lines as emission.
     
  16. Jul 15, 2017 #15
    Got it, thanks
     
  17. Jul 15, 2017 #16
    Above it was stated that when a piece of copper was inserted into a flame and the flame turned green it was due to the spectral emission of the copper. It was said that the copper vaporized where tungsten would not. Is this correct? If so, then if I had a piece of copper wire then why could I not heat it using electricity rather than a flame and get the green color.
     
  18. Jul 15, 2017 #17

    PeterDonis

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    Because the copper would melt and the wire would fall apart and stop conducting electricity, meaning the heating would stop.
     
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