Dinitrogen tetroxide, often represented by the chemical formula N2O4, is a chemical compound composed of nitrogen (N) and oxygen (O) atoms. It is an example of a nitrogen oxide and is closely related to the compound nitrogen dioxide (NO2). Dinitrogen tetroxide exists as a colorless to brownish-red gas at room temperature and pressure.

Dinitrogen tetroxide is an equilibrium mixture of two nitrogen oxides, nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4), due to its tendency to dimerize. The equilibrium between these two forms is influenced by factors such as temperature and pressure. When the temperature is lowered, the equilibrium shifts toward the N2O4 form, which is a colorless gas. When the temperature is increased, the equilibrium shifts toward the NO2 form, which is a reddish-brown gas.

The compound has several important applications and uses:

1. Rocket Propulsion: Dinitrogen tetroxide is used as an oxidizer in rocket propulsion systems. It is often used in combination with hydrazine-based fuels in hypergolic rockets, where the two substances ignite spontaneously upon contact, eliminating the need for an external ignition source.
2. Reagent in Chemical Synthesis: Dinitrogen tetroxide can be used as a reagent in chemical reactions, particularly in reactions that involve introducing nitro groups (-NO2) into organic molecules.
3. Air Pollution: In the atmosphere, dinitrogen tetroxide is a significant contributor to nitrogen dioxide levels, which can be harmful to human health and contribute to air pollution.
4. Oxidizing Agent: It can act as an oxidizing agent in certain chemical reactions.

Dinitrogen tetroxide is toxic and corrosive. It can cause severe respiratory and skin irritation upon exposure. It should be handled with appropriate safety precautions, including proper ventilation, protective equipment, and safe storage practices.