Question

Use your knowledge of periodic trends for the following questions! Do not look up the answers; trends are the important thing.

(a) Place the following elements in order of increasing size: Ca, Mg, Al, K.

(b) Place the following elements in order of increasing first ionization energy: As, Cl, Se, S.

Answer 1

(a) In general, atomic size decreases across the period and increases from top to bottom in a group.

Order of increasing size: Al < Mg < Ca < K

(b) In general, ionization energy increases across a period and decreases from to to bottom in a group.

As and Se are in same period and As followed by Se. So, As has less ionization energy than that of Se.

However, first ionization of As is larger than that of Se, because the stable half-filled electron configuration in As.

So, the order of increasing first ionization energy: Se < As < S < Cl