Argon is a chemical element, having the chemical symbol Ar. Its atomic number (the number of protons) is 18. It has a standard atomic weight of 39.948 g•mol −1, and is typically found as a gas in its elemental form.
Argon is a monatomic noble gas. At a pressure of 101.325 kPa, it has a boiling point −185.85 °C, and a melting point of −189.35 °C. It is the third most common gas in the Earth's atmosphere, at 0.934% by volume.