Hydrogen is the lightest element with the symbol "H" in the periodic table and has proton number 1. Hydrogen is one of the most abundant elements in the universe and is often found in large amounts in celestial bodies such as stars and gas giants. It is a very reactive element that can form various compounds with other elements. For example, water is formed by the combination of hydrogen and oxygen. Hydrogen is used as an important fuel and chemical feedstock used in many industrial applications. Hydrogen can be used to generate electricity in fuel cells and can be a clean energy source. Hydrogen is used in rocket fuels, the petrochemical industry, metallurgical processes, fertilizer production, and many other industrial processes. Hydrogen also plays an important role in the refining and hydrogenation reactions of iron, steel and other metals. The mixture of hydrogen gas with other gases can be explosive and must therefore be handled with care. In short, hydrogen is both one of the most abundant elements in the universe and plays an important role in a variety of industrial and energy applications.