Argon is a noble gas and has the atomic number 18. It is the third element in Group 18 of the periodic table. Noble gases are known for their stability and low reactivity. This is because their outermost electron shells are full, meaning that they do not readily gain or lose electrons.

In the case of argon, its outermost electron shell contains 8 electrons, which is the maximum number of electrons that this shell can hold. This full electron shell gives argon a very stable configuration, which means that it is unlikely to form chemical bonds with other elements. As a result, argon is found as a monatomic gas, meaning that it exists as individual atoms rather than molecules.

The other noble gases, helium, neon, krypton, xenon, and radon, are also monoatomic gases for the same reason. They all have full outermost electron shells, which makes them very stable and unlikely to form chemical bonds.