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Homework Help: Archived Atomic Spectrum of an Unknown Element

  1. Apr 8, 2013 #1
    1. The problem statement, all variables and given/known data

    When an electron drops from the M shell (n = 3) to a vacancy in the K shell (n = 1), the measured wavelength of the emitted x-ray is found to be 0.0897 nm. Identify the element.

    2. Relevant equations

    E = 1240 / λ
    E = (-13.6 / n^2) * (Zeff^2)

    3. The attempt at a solution
    According to my homework, the answer is Selenium (atomic number = 34). I initially got that answer by using: (-13.6 / 9) * Z^2 + (13.6 / 1) * Z^2 = 1240 / (.0897). Solving for Z here, I got 33.8.

    But I then learned that we are supposed to use Zeff, and not simply Z. I must be misunderstanding something though, because I would then think that the equation would be:

    (-13.6 / 9) * (Z - 10)^2 + (13.6 / 1) * Z^2 = 1240 / (.0897)

    because there are 10 electrons in the first and second energy levels, so I thought this equation would account for that. But I keep getting a Z value around 32.7 with this which would be in between Germanium and Arsenic.

    So what am I missing?
  2. jcsd
  3. Feb 7, 2016 #2
    The first thing about this problem is : Bohr's theory and his equations are not even valid here. This is not a one-electron system.

    As to how one does this, I am guessing Moseley's equation is the most valid.
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