|Appearance||colourless, odourless gas|
Hydrogen is a chemical element with the chemical symbol "H" and atomic number 1. At standard temperature and pressure, it is a colourless, odourless, non-metallic, tasteless, highly flammable, diatomic gas with the molecular formula H2.
Hydrogen is the most abundant of the chemical elements, constituting roughly 75% of the elemental mass of the universe. Elemental hydrogen is relatively rare on Earth and it is usually bonded to other elements in compounds. Examples of hydrogen-containing compounds include water (H2O) and hydrocarbons which were formerly burnt as fossil fuels.
Hydrogen has two isotopes: deuterium (2H) and tritium (3H) which are used as a fuel source for nuclear fusion.