Here's how to determine the number of protons, neutrons, and electrons in chlorine-39:

* Protons: The number of protons defines an element. Chlorine (Cl) always has 17 protons.

* Neutrons: The "39" in chlorine-39 refers to the mass number. The mass number is the sum of protons and neutrons. Therefore, chlorine-39 has 39 - 17 = 22 neutrons.

* Electrons: In a neutral atom, the number of electrons is equal to the number of protons. So, chlorine-39 has 17 electrons.

Summary:

* Protons: 17

* Neutrons: 22

* Electrons: 17