Nitric oxide (nitrogen oxide, nitrogen monoxide) is a molecular, chemical compound with chemical formula of NO that is a colorless gas under standard conditions. Nitric oxide is a free radical—i.e., its bonding structure includes an unpaired electron—and it is in the class of heteronuclear diatomic molecules that are of historic theoretical interest (for the insights they gave in formulating early modern theories of bonding). It is a practically important intermediate in the chemical industry. In addition, some is unavoidably produced during combustion of fossil fuels in power plants and automobile engines, with excess being created when there is present more air, or higher temperatures, than needed for efficient and complete combustion of the fuel. It is also produced naturally by the extremely high air temperatures produced along the path of lightning in thunderstorms.