We are creating the world's most trusted encyclopedia and knowledge base. Once you join us and log in, you'll be able to edit this page instantly!

From Citizendium, the Citizens' Compendium

Argon (symbol, Ar) is a chemical element with atomic number 18. It is an inert gas of the eighteenth group of the modern periodic table. It is present in a small quantity in the earth's atmosphere. It owes its non-reactivity mainly to its completely filled outermost shell.