Hydrogen is a chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element and most abundant chemical substance in the universe. Hydrogen has one proton in the nucleus and one electron in the outer shell. Hydrogen can form covalent bonds with other elements, including carbon, oxygen, nitrogen, and sulfur. Hydrogen is used in many industrial processes, including the production of ammonia, fertilizers, plastics, and fuels. Hydrogen is also used in fuel cells, which are devices that convert hydrogen and oxygen into electricity and water.