Question

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

Answer 1

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.