In English, to be inert is to be in a state of doing little or nothing.

Chemistry

In chemistry, 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 (such as combustion, for example) 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.

For example, 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 (Li₃N) 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.

Pesticides

The Federal Insecticide, Fungicide, and Rodenticide Act divides the ingredients in pesticides into two groups: active and inert. An inert chemical, under this context, is one that does not have a toxic effect on the species the pesticide is meant to combat, but that does not rule out that it may still have a biological activity on other species, including being toxic to humans. Solvents, propellents, preservatives, among others, are thus considered inert ingredients in pesticides. Inert ingredients in pesticide products. US Environmental Protection Agency, Office of Pesticide Programs.

Since 1997, the United States Environmental Protection Agency recommends that pesticide manufacturers label the non-active ingredients as "other ingredients" rather than "inert" to prevent misinformation to the public.

Number theory

In the branch of mathematics known as algebraic number theory, a prime ideal is said to be inert if it is still prime when considered in an extension field. Such a prime might have instead split as a product of other prime ideals, but by being inert it remains essentially unchanged.

Munitions

In the area of weapons, an inert munition is one in which all energetic material such as primer, fuse, and explosive or incendiary fill have been removed or rendered harmless. Inert munitions are used in military training and are collected in museums and by private individuals. See also military dummy. Typically, inert munitions are painted entirely in light blue and/or have the word "INERT" stenciled on them in a prominent position.