The element that bonds with itself to form the simplest of molecules is hydrogen. Hydrogen molecules (H2) consist of two hydrogen atoms covalently bonded together. These molecules are the most abundant in the universe, making up about 90% of all atoms. Hydrogen molecules are highly flammable and have the highest thermal conductivity of all gases. They are also used in a variety of industrial processes, including the production of ammonia, fertilizers, and fuels.