Identify the number of protons given in the problem, which is 17. The number of protons defines the element's atomic number.
Recall that the element with 17 protons is chlorine, since chlorine's atomic number is 17.
Calculate the mass number of the isotope by adding the number of protons and neutrons: \(\text{mass number} = \text{protons} + \text{neutrons} = 17 + 18\).
Match the calculated mass number with the isotope notation. The isotope is written as Element-MassNumber, so here it would be Chlorine-35.
Compare this with the given options to confirm that Chlorine-35 is the correct isotope with 17 protons and 18 neutrons.