Here's why:

* Atomic Number: The atomic number defines an element and represents the number of protons in the nucleus of an atom. All chlorine atoms have 17 protons.

* Mass Number: The number "37" in chlorine-37 is the mass number, which represents the total number of protons and neutrons in the nucleus.

So, while the mass number changes with different isotopes, the atomic number (and therefore the element) remains the same.