Hydrogen is a gas at room temperature and pressure. It is the lightest and most abundant element in the universe, and is colourless, odourless, non-metallic, and highly flammable. Hydrogen is a diatomic molecule, meaning it consists of two hydrogen atoms bonded together. It is the most reactive of all elements, and can react with almost any other element to form compounds. Hydrogen is used in a wide variety of applications, including as a fuel for rockets, fuel cells, and in the production of fertilisers and plastics.