Explain how chemists can state with certainty that no one will discover an element that would appear on the periodic table between sulfur (S) and chlorine (CI)

Sulfur has 16 protons and chlorine has 17 protons. Since the placement on the periodic table is equal to the number of protons, and also since it is impossible to have an element with a non-whole number of protons, there can be no element between sulfur and chlorine.