Question

Why is the ionization energy of CI lower than F, but higher than that of S?

Short answer

The ionization energy of chlorine is lower ionization energy than Fluorine because it is below $F$, and ionization energy of $Cl$ is higher than$S$ because it is to the right of $S$.

Step-by-step solution

Given Information 

We need to find the ionization energy of CI lower than F, but higher of S.

Explanation 

Ionization energies decrease going down a group, but going across a period denotes an increase in ionization energies.

In according their positions in the periodic table, Chlorine has lower ionization energy than Fluorine because they belong in the same group, and Chlorine is below Fluorine.

Next, Chlorine has lower ionization energy than Sulfur because they belong in the same period, and Chlorine is at the right of Sulfur.