Hydrogen (H) is a chemical element with atomic number 1. It is a colorless, tasteless, odorless substance, nontoxic but highly combustible, and is a gas under standard observational conditions. Hydrogen is the most lightweight and abundant element on the periodic table, constituting three-quarters of all the normal baryonic matter in the universe. A single proton is considered a hydrogen atom, whether or not it is neutrally charged by the accompaniment of an electron; therefore, hydrogen atoms appeared within the first second of the universe’s existence. Stable, neutrally-charged hydrogen atoms would not appear until 370,000 years later when the universe had cooled down enough for electrons and protons to bind to one another.
