The correct order of ionic radii for the ions P, S, Ca, K, Cl is:

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Strategy:
 Use the principle that for a set of isoelectronic ions, the size increases with decreasing nuclear charge.

Step 1: Identify isoelectronic ions
All ions () have 18 electrons.

Step 2: Compare nuclear charges
-  ()
-  ()
-  ()
-  ()
-  ()

Step 3: Order by size
Since size , the order is:

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