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.