A chlorine atom with 17 protons and 18 neutrons is called chlorine-35 or ³⁵Cl.

Here's why:

* Protons determine the element: The number of protons (17 in this case) defines an atom as chlorine.

* Neutrons determine the isotope: The number of neutrons can vary within an element. This variation creates different isotopes of the same element. Since this atom has 18 neutrons, it's an isotope of chlorine called chlorine-35 (the atomic mass is the sum of protons and neutrons).