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# Rank the elements in each of the following sets in order of decreasing First Ionization Energy IE$_1$: K, Ca, Rb

$\begin{array}{1 1} Ca > K > Rb \\ K > Rb > Ca \\ Ca > Rb > K \\ Rb > Ca > K \end{array}$

Answer: Ca > K > Rb
IE decreases as you proceed down in a group; IE increases as you go across a period.
Ca is to the right of K; Rb is below K.