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:
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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