A molecule which consists of two atoms is known as a diatomic molecule. At room temperature, virtually all diatomic molecules are gases. But some substances that have crystalline or other atomic arrangements at room temperature become diatomic at higher temperatures. A few compounds such as carbon monoxide, hydrogen chloride and nitric oxide have diatomic molecules. The Earth’s atmosphere is mainly composed of the diatomic molecule’s oxygen (O2) – about 21% and nitrogen (N2) – about 78%. Hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), and chlorine (Cl2) – occur as diatomic elements at room temperature, which is 25 degrees Celsius.