Chlorine with mass numbers 35 and 37 are the same element because they have the same atomic number (17).

Here's why:

* Atomic number defines the element: The atomic number represents the number of protons in an atom's nucleus. Protons determine the element's identity. All chlorine atoms, regardless of their mass number, have 17 protons.

* Mass number is the total number of protons and neutrons: The mass number can vary because the number of neutrons can change. This leads to isotopes.

So, while chlorine-35 and chlorine-37 have different numbers of neutrons (18 and 20, respectively), they both have 17 protons, making them both chlorine.