if element 17 in a halogen where you would expect to find element 18?

If element 17 is a halogen, element 18 is expected to be a noble gas.

This is because in the Periodic Table, a halogen is followed by a noble gas.

As atomic number 17 is halogen seats in group 7A in periodic table, the next element 18 is in group 8 as a inert or noble gas, which has 8 electron in it's outermost orbit.

In the noble gases.
Atomic (Element) No. 17 is Chlorine(Cl)
Atomic (element) No. 18 is Argon (Ar)

inert gas group

Noble gas