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Homework Help: Calculating effective nuclear charge

  1. Oct 22, 2008 #1
    1. The problem statement, all variables and given/known data

    If ionization energy is 899.4 kJ/mol for Be, what is the effective nuclear charge?

    2. Relevant equations
    Zeff = Z - S

    E=RH(Z2/n2) ??

    3. The attempt at a solution

    My attempted solution was subbing into
    Zeff = Z - S
    Zeff = 4 - 2
    = 2

    But I suspect that is wrong... because why ionization energy is given.. so shouldn't it be used in the calculation?

    And somewhere I think I read that "S" was supposed to be a "constant" of some sort, and I just subbed 2 in because I thought that it was the number of electrons in the first orbital ?
  2. jcsd
  3. Oct 23, 2008 #2


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    Staff: Mentor

    One of the equations you listed contains both ionization energy and effective nuclear charge, why don't you use it?
  4. Oct 23, 2008 #3
    So subbing in values
    E=899.4 kJ/mol
    RH=2.178 x 10-21 kJ

    I get Z2eff= 4.129 x 10 23 mol

    How does one get to the units/value of Z2eff after this?
  5. Oct 23, 2008 #4


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    Staff: Mentor

    Ionization energy was per mole, not per molecule.
  6. Oct 23, 2008 #5
    Oops, just noticed that "Z2eff= 4.129 x 10 23 mol " should actually read "Z2eff= 4.129 x 10 23 mol-1 "

    I think that the italicized part is what confuses me the most -- what are the units for this portion? I'm going to take a guess that it is currently molecules/mol, but if so, is this always the case whenever expressing a value and the unit mol-1?

    Like for this example, what was given was in kJ/mol. When the kJs were cancelled, what resulted was just mol-1...
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