K < Na < Al < Si < Cl < Ar
Strategy and Explanation Since ionization energy increases across a period, we expect the ionization energies for Ar, Al, Cl, Na, and Si, which are all in Period 3, to be in the order Na < Al < Si < Cl < Ar. Because K is below Na in Group 1A, the first ionization energy of K is less than that of Na. Therefore, the final order is K < Na < Al < Si < Cl < Ar.