Which of the following ions has the smallest radius: Ca2+, K+, F-, or Cl-?

Question

Would it be K+, because it is a cation, making it have a smaller radius?

Answer 1

Look at the electron configurations.
For example, between K^+ and Ca^+2.
K is 1s2 2s2 2p6 3s2 3p6 4s1 which makes
K^+ 1s2 2s2 2p6 3s2 3p6 and
Ca^+2 1s2 2s2 2p6 3s2 3p6
but K^+ has 19+ charges tugging.
Ca^+2 has 20+ charges tugging on the same number of electrons and in the same shells; therefore, Ca^+2 must be smaller than K^+.
Now look at F^- and Cl^- and go through the same, then compare with these + charged ions.