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Having a problem with atom/ion radius

  1. Jan 20, 2005 #1
    Okay I think this question has been addressed slightly before however I seem to be having difficulty on it still.

    The question asks: Arrange the following atoms or ions in order of increasing radius Cl, S2-, K, K+, O

    Now I have them arranged as

    O < Cl < K+ < S2- < K

    The arrangement of K+ and S2- is where I am stumped. The question goes on to ask "Give an explanation for the position of S2- in relation to the atom or ion that comes just before and just after" Now I would only assume they're talking about the K+ and K atom/ion so I placed S2- in between. Now my best guess at why S2- is where it is, is due to the gain in electrons. But how do I prove this? Is it because the S atom has gained two electrons and the K atom has only lost one that the S2- ion becomes larger than the K+ ion?

    Is there a certain method I should be using to determine the actual resultant size of these atoms when they gain or lose electrons?

    I tried to keep the post as informative as possible I'm not trying to scam any answer out of anyone I'm trying to learn how to solve this problem.
  2. jcsd
  3. Jan 20, 2005 #2


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    Hello, this website mentions about sulfur's neutral and anionic radii. Potassium atom has the radius of 2.27 angströms, and 1+ cation has 1.52 angströms (according to the same website, change the final part as potassium.htm). Since S2- is 1.70 angströms, it is much larger than potassium cation. The best explanation would be listing their respective radii with them, the sorting will automatically be done.
  4. Jan 20, 2005 #3

    Only problem is I believe that stating the exact atomic radius is beyond the scope of the question and this part (if not all) of the course. I also believe that just simply stating their atomic radius does not explain fully why S2- is located where it is.
  5. Jan 20, 2005 #4


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    First let's prove that : K+ < S2-

    These two are isoelectronic species (they both have 18 electrons). So, the difference between their ionic radii comes down to the difference in their nuclear charge. K+ has 19 protons while S2- only has 16. Due to the larger nuclear charge, K+ must necessarily be smaller (as the attraction towards the nucleus is greater).

    Now compare K and S2-

    K must be bigger because you are just starting to fill the 4th shell, while S2- only has electrons in 3 shells. However as you go along the period from K to Ca, Sc, etc. the radius decreases making the judgement difficult. This argument is not complete, as it does not compare nuclear charges. While S2- has a smaller nuclear charge thanK, the increase in radius due to this, is not sufficient to overcome the incresing size due to the addition of a new shell. In fact, P3-, which would be bigger than S2- (by the isoelectronic species argument), is still smaller than K.

    Warning : Don't not use the Isoelectronic Species comparison to the Noble Gases. They can be weird ! :yuck:
  6. Jan 21, 2005 #5
    wow man thanks for the help here... this question was getting to me

    one of those questions i guess when you finally get the answer its like "What the hell i should have known that!"
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