Element occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. represented by the atomic symbol Ar, that has Z = 18, and an Mass of a neutral atom of a nuclide - also called "atomic weight." The atomic weight of an element is the weighted average of each isotope. of 39.948. It is colorless, odorless, and very inert gas, comprising ~1 % of the Earth’s atmosphere.