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The term inert is used to describe something that is not chemically active. The noble gases were described as being inert because they did not react with the other elements or themselves. It is now understood that the reason that inert gases are completely inert to basic chemical reactions is that their outer valence shell is completely filled with electrons. With a filled outer valence shell, an inert atom is not easily able to acquire or lose an electron, and is therefore not able to participate in any chemical reactions. For inert substances, a lot of energy is required before they can combine with other elements to form compounds. High temperatures and pressure are usually necessary, sometimes requiring the presence of a catalyst.

In petroleum marketing operations, another use of the term inert has to do with the testing of a tank or piping for leaks. Some tightness-testing methods call for introduction of inert gases, under pressure, into a closed tank or piping system.

Elemental nitrogen is inert under standard room conditions and exists as a diatomic molecule, N2. The inertness of nitrogen is due to the presence of the very strong triple covalent bond in the N2 molecule; nitrogen gas can, however, react to form compounds such as lithium nitride under vigorous conditions.

Inert atmospheres of gases such as dinitrogen and argon are routinely used in chemical reactions where air sensitive and water sensitive compounds are handled.