How can chemists state with certainty that no one will discover an element that would appear on the periodic table between sulfur (S) and chlorine (Cl)?
Do chemists state this with certainty?
Given what we know now, namely that elements contain different numbers of fundamental particles, I expect chemists (and more so physicists) would be extremely surprised if a new element appeared between sulfur and chlorine. A lot of them would not believe it.
Our current understanding of the elements proposes that elements differ on the basis of the number of the fundamental, charged particles they contain. Sulfur contains 16 massive charged particles, and 16 oppositely charged particles of negligible mass. Chlorine contains 17 massive charged particles, and 17 oppositely charged particles of negligible mass. Of course each nuclide contains 16 or so massive particles of neutral charge, but this really doesn't affect the chemistry very much.
A new element between sulfur and chlorine would overturn the basis of physics and chemistry. It would launch a new paradigm.