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Isoelectronic series?

  1. Mar 31, 2005 #1
    15. The following ions comprise an isoelectronic series: S2, Cl, K+, and Ca2+
    Which is the largest ion?

    A. S2-
    B. Cl-
    C. K+
    D. Ca2+
    E. All should be the same size.

    Don't even know what one is.
  2. jcsd
  3. Mar 31, 2005 #2
    What are your periodic trends for atomic size?
  4. Mar 31, 2005 #3
    Atomic radius increases to the left and down.
  5. Mar 31, 2005 #4
    Does the term "isoelectronic series" do anything for this problem?
  6. Apr 1, 2005 #5


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    Staff: Mentor

    It just means that all ions have exactly the same number of electrons, the only thing that changes is nucleus charge.
    Last edited by a moderator: Aug 13, 2013
  7. Apr 1, 2005 #6


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    It quite self-explanatory, atoms with the same number of electrons. You should be able to figure out the electron number in a neutral atom of the element, and further deduce the electron number of a corresponding ion.

    In reference to the size of each of these, you'll need to consider the relative proton number with the electron number, effective nuclear charge. Typically, a isoelectronic cation is smaller than the anion because you've got the same number of electrons for both with a smaller proton number for the latter.
  8. Apr 1, 2005 #7


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    Staff Emeritus
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    Gold Member

    Not to blow any trumpets here but this essentially summarizes what has been said above by Borek and GenChemTutor, about comparing radii among isoelectronic ions.
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