Chemical inertness usually arises for one of two distinct scientific reasons. The first reason is a highly stable electron configuration within the atoms of the substance. The noble gases exhibit this natural stability because they possess completely filled outer valence electron shells.
The second reason is a high activation energy barrier that forcefully blocks a potential chemical reaction. This kinetic barrier prevents the reaction from starting even when it would be thermodynamically favorable. Because of these varying factors, inertness is considered a relative concept rather than an absolute rule.
Given enough external energy or extreme environmental conditions, many supposedly inert substances can actually be forced to react. Adding a suitable chemical catalyst can also cause an inert substance to suddenly begin reacting rapidly. Chemists frequently use the word inert to describe how materials behave under normal daily laboratory conditions.
Inert materials are widely used to protect sensitive chemical reactions from unwanted outside interference. An inert gas can easily prevent oxygen or water vapor from destroying a very delicate chemical sample. Inert glass containers are used to safely store corrosive acids because the glass will not react with them.
Engineers also use inert materials to build chemical manufacturing plants that must safely handle aggressive reagents. Teflon is a popular inert plastic used to coat internal pipes and valves in these extreme industrial environments.