Air is mostly nitrogen and oxygen.
Each blue ball represents an atom of nitrogen and each red ball represents an atom of oxygen. In air, most molecules of both nitrogen and oxygen contain two atoms. The flames inside boilers and car engines reach such high temperatures that the nitrogen and oxygen in the air combine to form nitric oxide. The nitric oxide quickly combines with more oxygen in the air to form nitrogen dioxide. In warm, sunny air, the nitrogen dioxide breaks down into a molecule of nitric oxide and an atom of oxygen.