Question

Which of the following pairs of elements would you expect to have lower first ionization energy? Explain.

(i) Cl or F

(ii) Cl or S

(iii) K or Ar

(iv) Kr or Xe

Answer 1

The ionization energy.

(a) Decreases in going from top to bottom in a group.

(b) Increases in going from left to right in a period.

Thus

(i) Cl is below F in the group (or Cl is bigger than F), hence Cl has lower ionization energy than F.

(ii) Cl is in the group 17th, while S belongs to 16th group (of the same period, Period 3). Thus, S should have lower ionization due to lower nuclear charge.

(iii) K appears immediately after Ar in the periodic table. K belongs to group 1 of the periodic table; hence K should have lower ionization energy.

(iv) Xe occurs below Kr in group 18 of the periodic table, hence Xe should have lower ionization energy due to its larger size.