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

Because there can be no element with such an atomic number between them that it constitutes a new element. There can be no element that has 16.5 protons because protons are single, indivisible units. There can be a different number of neutrons however but this would only create an isotope of the same element.

Biased means unfair but that's all ik