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The Truth about the Atom

  1. Jan 31, 2016 #1
    Okay guys

    So I was reading physical chemistry by atkins and quantum chemistry by ira.n.levine
    But there's one thing i am not quite clear with

    So it's like this:

    I do know that an atom has a definite energy(which is the sum of its potential (which includes the potential and(energy due to interelectronic repulsions and nucleus-electron interactions which i shall tend to neglect for the time being)and kinetic energies(here kinetic energy includes only the transational kinetic energy of the atom)

    And let's say that the atom is a multi-electron one
    Now let's say we provide some energy to the atom making it excited or allowing it leave its ground state and reach an excited one

    As a consequence of this,one of the electrons in its valence shell gets excited(i can't pinpoint which since the excitation of any one electron is a probability in itself)
    this electron after going to its excited state returns to its ground state by releasing a certain quanta of energy
    Corresponding to a certain frequency

    Now herein lies my question
    As we know that electron transfer is feasible for atoms in gaseous state(I am not talking about the photoelectric effect nor am i highlighting the dissociation of an ionic compound/acid/base into cations and anions)

    And we also know that gaseous atoms also possess some thermal energy due to their random motion(which serves as a basis for the total internal energy of the system)
    So let's say we energise a gaseous atom!

    So will all that energy it receives go into energising its outer shell electron
    Or will the energy be expended in parts(some amount of the energy excites the electron,while the rest of it increases the kinetic energy of the atom)

    I do know that when you energise the atom
    You energise the entire nucleus+ electrons system
    And thus can't we say that the energy of the entire atom increases as a result of which its kinetic energy also increases and that too by certain discrete amounts since the energy of the atom is quantized
    Some insight is much appreciated!!!!


    And could you guys give me a more word/logic-based explanation instead of a numerical one??
    My stupid brain refuses to understand quantum mechanics completely

    Last edited by a moderator: Jan 31, 2016
  2. jcsd
  3. Jan 31, 2016 #2


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    Well, momentum has to be conserved, and the absorbed photon carries some momentum. It will change the motion of the atom. It can increase its kinetic energy or decrease it, depending on its flight direction relative to the direction of the absorbed light.

    This is relevant for precision spectroscopy: Atoms that move towards your light source get slowed down and lose some kinetic energy, and the photon needs a lower energy for the excitation. Atoms that move away from your light source get sped up and gain some kinetic energy, and the photon needs a higher energy for the excitation. Overall, this is called "doppler broadening" of spectral lines.
  4. Jan 31, 2016 #3
    Photon needs a lower energy for excitation??
    How can photons get excited
    Aren't they just light quanta??
    Do you mean the electrons??

  5. Jan 31, 2016 #4


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    The photon needs a lower energy for the excitation of the atom.
  6. Jan 31, 2016 #5
    Your last 4 questions relate esp. to lasers. Read a book on that basics before ocupying Yourself with quantum chemistry. In that field, electrons will dance weird orbitals esp. when " the atom is a multi-electron one"

    TheTruth is, nobody "understands complete" QM, because of its weird quantum logic which requires abstract and new mathematical concepts of superposition states beyond normal imagination. QM objects don´t follow classical concepts, nevertheless there are rigid rules and laws, it can be calculated in some fields precisely and it works.
  7. Feb 1, 2016 #6
    Ummm okay
    Could you suggest some good books to me about lasers?
  8. Feb 1, 2016 #7


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  9. Feb 1, 2016 #8
    I mean will it do that for sure???
    If yes then thank you so much for allowing me to get rid of my confusion!!!!!:)

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