The isotope chlorine-37 (denoted as 37Cl) has 17 protons, 20 neutrons, and 17 electrons. The number of protons determines the element's identity, which in this case is chlorine (atomic number 17). The sum of protons and neutrons gives the mass number, which in this case is 37. Electrons in a neutral atom are equal to the number of protons to balance the charge.