Final answer:
No element will be discovered between sulfur and chlorine on the periodic table because each element is defined by its distinct atomic number, which cannot be fractional, and the atomic number for sulfur is 16 and for chlorine is 17.
Step-by-step explanation:
Chemists assert with certainty that no new element will be discovered between sulfur and chlorine on the periodic table due to the well-established basis of the table's arrangement.
Each element on the periodic table is uniquely defined by its atomic number, which represents the number of protons in the nucleus of an atom. Since sulfur has an atomic number of 16 and chlorine has an atomic number of 17, there can be no element with an atomic number between these two. Furthermore, the chemical properties of elements are determined by their electron configurations, specifically in the valence shell, and since these configurations are tied directly to atomic numbers, it would be impossible for an element to exist between sulfur and chlorine as it would necessitate a fractional atomic number, which does not occur in nature.