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. Click on Term to Read More 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. Click on Term to Read More of 39.948. It is colorless, odorless, and very inert gas, comprising ~1 % of the Earth’s atmosphere.