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Quantum mechanics replacing bohr model

  1. May 18, 2009 #1
    do all of the electrons in an atom have to move up or down an energy level or is it just the valence electrons or what? is there one photon released per electron and are they all the same wavelength?
     
  2. jcsd
  3. May 19, 2009 #2
    You are trying to ask something like. "I heard an explanation on how atoms emit light. I understand it has to to with electrons jumping from one energy state into another. I am confused as to which atoms participate in this process, and how many photons are emitted."
    I'll answer anyways: It doesn't matter which electron changes its energy state, when it does one photon is released carrying away the energetic difference.
     
  4. May 19, 2009 #3
    but if only one electron did this and changed energy levels wouldnt it change the chemical properties of the atom? for instance, if one electron in helium moved up an energy level it wouldnt be stable anymore and it would have different chemical properties
     
  5. May 19, 2009 #4

    alxm

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    It would have different chemical properties, yes. I'm not sure what you mean by 'stable' - atoms are quite stable. But if you excite (move up in energy) an electron with enough energy (the ionization potential), it may leave the atom altogether.

    If you're referring to molecules, then yes - exciting an electron can destabilize it, causing it to fall apart (photodissociation). Or form a bond. Most molecules are, in most cases, stable. But for example: if UV light hits an electron in the double-bond in a thymine base-pair in your DNA, it'll temporarily break that bond - at which point the thymine might react by forming a bond to a neighboring thymine instead - causing what's called a 'thymine photodimer'. Which in turn distorts the shape of the DNA double-helix, which means the DNA of that cell stops working correctly, and if you're real unlucky, you get a melanoma. (now that summer is on its way, a skin-cancer warning would be appropriate - wear sunblock all you pasty white physics geeks! :D)

    But at ordinary temperatures, electrons don't stay in excited states for very long. (how long does your average object glow after you turn out the lights?) So you don't usually talk about chemical properties of excited molecules - since they can't be kept around and chemically isolated and so on. Rather you just talk in terms of which photoreactions, if any, can take place.
     
  6. May 19, 2009 #5
    ah i see ok ty and i meant chemically stable but i have another question; if you rub two objects together underwater they can keep putting out heat energy forever; once all of the electrons are in their lowest level wouldnt the heat energy stop being put out? or does the kinetic energy from moving the objects cause some of the electrons to move up an energy level?
     
  7. May 19, 2009 #6
    When you rub two objects together and generate heat from the friction, the energy comes from you, and is transformed into vibrational/heat energy in the object. You can't cool an object by rubbing it. Is this what you meant?
     
  8. May 19, 2009 #7
    oooh ok i get it now i was thinking that when you rubbed two things together their electrons interacted and emitted infrared radiation but i take it that they interact without dropping energy levels by emitting virtual photons and cause the opposite object's atoms to move erratically or move however solids do when they're hot
     
  9. May 20, 2009 #8
    Here's what happens when you rub two objects together:

    1. You add kinetic energy to the object by rubbing the objects together, striking them together, etc. The atoms in the object gain kinetic energy. Soon enough this internal kinetic energy reaches thermal equilibrium. If you dropped the object on the ground, you might have vibrations of a certain frequency and a large amplitude ringing throughout the object, especially if it's metal and the ground is hard, but eventually that one frequency component with a big amplitude becomes a distribution of frequency components with small amplitudes. This is thermal energy.

    2. All objects emit thermal radiation, which consists of light in a distribution of many different frequencies. The higher the temperature, the higher the frequency that's being emitted the most. At room temperature, the peak amplitude is in the infrared, but if we heat something up enough, it'll be glowing hot, and the peak amplitude will be in the visual frequencies.

    This is not to say the electrons from different surfaces don't interact with each other and emit stuff. I mean, if you can peel Scotch tape in a vacuum and create x-rays, then it seems anything's possible. Nevertheless, you don't need to consider the surface interactions to explain why rubbing objects produces heat.
     
  10. May 20, 2009 #9

    alxm

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    No, not with normal temperatures. The states that normally correspond to heat is the kinetic energy of the atoms/molecules (their translational speed), their rotational energy levels, and their vibrational energy levels. (they have discrete levels just like the energy levels of the electrons).

    Heat can cause electronic excitations, but only at very high temperatures.
     
  11. May 20, 2009 #10
    ooh ok i get it thanks guys!
     
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