The two crossword solutions for the term 'prefix for element #9' are 'Fluor' (5 letters) and 'Fluoro' (6 letters). Fluorine is the chemical element with atomic number 9, and both 'Fluor' and 'Fluoro' are prefixes used to denote the presence of fluorine in a compound. The prefix 'Fluor' is derived from the Latin word for fluorine, 'fluorum', while 'Fluoro' is derived from the Greek word for fluorine, 'phluoros'.