Are all elements in group 17 metals?
The halogen group, given their nuclear charge, is highly oxidizing. Fluorine in particular is the most reactive element on the Periodic Table. Of course this is a result of electronic structure.
Given that the halogens are oxidizing species, and as elements are (i) molecular, and (ii) gaseous or liquid (well at least fluorine, bromine, and chlorine are!), the halogens are NON-metals............