An inert gas is a gas which does not undergo chemical reactions under a set of given conditions. The noble gases and nitrogen often do not react with many substances. Inert gases are used generally to avoid unwanted chemical reactions degrading a sample. These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in air. The term inert gas is context-dependent because nitrogen gas and several of the noble gases can be made to react under certain conditions. Purified nitrogen and argon gases are most commonly used as inert gases due to their high natural abundance (78% N₂, 1% Ar in air) and low relative cost. Unlike noble gases, an inert gas is not necessarily elemental and is often a compound gas. Like the noble gases the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases. This is a tendency, not a rule, as noble gases and other "inert" gases can react to form compounds