Argon (Ar) is a chemical element with atomic number 18. It belongs to a group of elements called noble gases. Making up roughly 1% of the Earth's atmosphere, argon must be considered a fairly abundant gas. There is more than twenty times more argon than carbon dioxide (CO₂) in our air.

By far the most common argon isotope on Earth is ⁴⁰Ar, with 18 protons and 22 neutrons. Interestingly, the abundance of this heavy isotope gives argon a higher atomic weight than potassium which is the next element in the periodic table. As a rule, atomic weight increases with atomic number, but not in this partiular case.