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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

$\begin{array}{1 1} Xe > I > Cs \\ I > Cs > Xe \\ Xe > Cs > I \\ Cs > I > Xe \end{array}$

Answer: Xe > I > Cs
IE decreases as you proceed down in a group; IE increases as you go across a period.
I is to the left of Xe; Cs is furtther to the left and down one period.