Inert gases are completely inert to basic chemical reactions (such as combustion, for example) because their outer valence shell is completely filled with electrons. With a filled outer valence shell, an inert atom is not able to acquire or lose an electron, and is therefore not able to participate in any chemical reactions. For inert atoms or molecules, a lot of energy is involved before it can combine with other elements to form compounds. A high temperature and pressure is necessary, and sometimes requires the presence of a catalyst.