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Questions on Line Spectra, Orbitals and Radiation

  1. Dec 19, 2007 #1
    I have several questions that I would appreciate if you guys could help me with, I want to understand these conepts as well as possible.

    In my chemistry textbook it states that the atoms of each element have a spectral signature, and each atom emits a certain wavelength of radiation, for example hydrogen atoms when excited emit blue light. I thought that radiation was emitted across the spectrum, and only some ranges of the spectrum was emitted more than others. Is the book saying that hydrogen only emits radiation in the range of blue visible light?

    Also, the book says that the intensity we perceive from radiation has to do with the amplitude of the wave, but later in the chapter, it says the perceived difference in intensity between a dim light and a blinding light has to do with the frequency. Is there a way to correlate the two definitions of intensity?

    I am starting to understand the conept of orbitals being different energy levels I believe. According to my understanding, if an atom has only one electron, it will be held close to the nucleus in what we call the 1s orbital. If you add more electrons you add to an atom, due to electron-electron repulsion, some electrons will be held farther from the nucleus in outer orbitals, which will not be held as tightly. In order for one atom to move to a higher orbital, it must receive an amount of energy which is equal to the attraction it has with the nucleus at that particular orbital. Is this correct?

    Last question, if an atom is excited from the first orbital to the third orbital for example, does it always return to the first orbital, or can it move back to say the 2nd orbital?

    Thanks in advance for any help
     
  2. jcsd
  3. Dec 19, 2007 #2
    Hydrogen and other atoms emit at particular wavelengths that are determined by the diffferences in energy between the levels that the electrons go between. A photon of radiation is emitted when the electron goes to a lower energy level. When an electron goes to a higher energy level a photon is absorbed. In either case the frequency of the radiation, times Planck's constant, is equal to the difference in energy between the two levels of the electron. Hydrogen can emit radiation at many places in the spectrum, but only at specific wavelengths.

    The intensity of the radiation is related to the amplitude of the wave (the square of the amplitude is proportional to the number of photons). As for dim and bright lights, the author probably is talking about the light getting hotter when it gets brighter. A hotter filament emits more short-wavelength light. This is a temperature effect. Of course, hotter doesn't always mean brighter. I can see why this would be confusing.

    I hope your question about orbitals is answered above. Pertaining to your last question, if an electron in one level is excited to another level, it doesn't necessarily
    return to the first level. Many sequences of transitions are possible. However, if it starts in the lowest energy level of the atom it will probably eventually end up back there, since in a higher level it will tend to emit a photon and go to lower levels until the lowest level is reached.

    Note that not only atoms emit and absorb radiation. Molecules do it too. And its not always electrons making the energy transitions. There are other ways to change energy.
     
  4. Dec 20, 2007 #3
    Not quite, in order to move from orbital 1 to orbital 2, the electron must absorb and amount of energy equal to the potential energy difference between orbital 1 and orbital 2.
     
  5. Dec 20, 2007 #4
    Thanks for the replies

    So what does it mean to say one orbital has more energy than another? For example, that the 3s orbital has a higher energy than the 2s orbital. The electrons in the 2s orbital are held more tightly to the nucleus than the electrons in the 3s orbital so wouldn't the 2s orbital have a higher energy? I feel like I am missing something here.
     
  6. Dec 20, 2007 #5
    When the electron is lower in the potential well it has less energy.
     
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