Question

How do you arrange the elements Cl, S, Sn, and Pb in order of decreasing first ionization energy?

Answer 1

You consult tables of ionization energy and arrange the elements accordingly.

Explanation:

Ionization energy increases ACROSS a Period (from left to right as we read the Table), and decreases DOWN a Group. You understanding of electronic structure should be good enough to rationalize these phenomena.

You have drawn elements from different Periods and from different Groups, so there is no apparent trend. Chlorine and sulfur come from the 3rd Period, and since chlorine is to the right of sulfur on the Table, it will display a higher 1st ionization energy. As a chemist you should actually put numerical values forward.