Argon (Ar). Diagram showing the nuclear composition and electron configuration of an atom of argon-40 (atomic number: 18), the most common terrestrial isotope of this element. The nucleus consists of 18 protons (red) and 22 neutrons (yellow). 18 electrons (white) bind to the nucleus in shells (rings), filling the outer (third) electron shell in what is a very stable configuration. The stability of an element's outer electrons determines its chemical and physical properties. Argon is a noble gas in group 18, period 3, and the p-block of the periodic table. An inert gas formed from the decay of potassium-40, it is used in fluorescent lighting and lasers.