I looked up the neutral atom and ionic species and this is what I found from smallest to largest. It isn't what I expected.
Ar < Cl^- < Ca^2+ < K^+
I thought in an isoelectronic series (which this is since all atoms/ions have 18 electrons) that Cl^- would be larger than Ar because you are adding an extra electron to make the -1 ion, the electrons repel each other, and that makes the size larger. On that basis then, Ar, the next element after Cl will be slightly smaller because there is an extra + charge in the nucleus (18 vs 17) and that pulls the outside electrons in a little more. So far so good. That makes sense and is true. Ar is slightly smaller than Cl^-. Then my reasoning goes that the K^+ should be slightly smaller than Ar because the same number of electrons are there and there is one more proton in the nucleus. That should pull the electrons in more and the trend to Ca^2+ should continue BUT that doesn't fit the data.
That is the trend for O^2-, F^-, Ne, Na^+, Mg^2+. I don't know why it isn't the trend in your series.
Here is a site showing the trend in decreasing size for an isoelectronic group. Scroll down part way to see the diagram. http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends_in_Ionic_Radii
Here is the site where I picked up the atom/ion sizes. Scroll to almost the bottom of the page.
http://www.angelo.edu/faculty/kboudrea/periodic/trends_atomicradius.htm
Define isoelectronic series. Put the K+, Ca2+, Cl-, and Ar ions and atoms in order of increasing size and explain the trends.
Count the number of electrons for each atom
Then, explain the (column) size trend for Mg2+ versus Ca2+ ions
Is the order cl k ca Ar? How would you count electrons? What would the size trend be?
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