Molecular nitrogen (dinitrogen, N=N) makes up about 78 percent of the atmosphere. It is the most unreactive diatomic species known. Interestingly, however, nitrogen is required for all life; it is used to build proteins and DNA. Therefore, dinitrogen must be turned into a molecule that can be assimilated readily by plants. That molecule is ammonia, NH₃.

Prior to World War I, the iron-catalyzed Haber-Bosch process for ammonia synthesis at high temperatures (350 to 550 °C) and pressures (150 to 350 atmospheres) from dinitrogen and dihydrogen (H₂) was discovered. It is perhaps the most important industrial process ever developed and responsible for a dramatic increase in the population of the earth during the 20th century, because it supplies a reliable source of nitrogen for fertilizers. But because the Haber-Bosch process requires high temperatures and pressures, it consumes tremendous amounts of energy; it is estimated that as much as 1 percent of the world's total energy consumption is devoted to the process.