Using only a periodic table, arrange these atoms in order of increasing first ionization energy: Al, Ar, Cl, Na, K, Si.
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.