* Find the atomic numbers: The number of protons in an atom is its atomic number.

* Chlorine (Cl) has an atomic number of 17.

* Potassium (K) has an atomic number of 19.

* Identify the element: We need an element with more protons than chlorine (17) but fewer than potassium (19). The only element that fits this description is Argon (Ar), which has an atomic number of 18.