Here's how to determine the mass number and atomic number of a chlorine atom with 17 protons and 18 neutrons:

* Atomic Number: The atomic number is the number of protons in an atom's nucleus. Chlorine always has 17 protons, so its atomic number is 17.

* Mass Number: The mass number is the total number of protons and neutrons in an atom's nucleus. In this case, it's 17 protons + 18 neutrons = 35.

Therefore, this chlorine atom has an atomic number of 17 and a mass number of 35.