Hydrogen (chemical symbol H) is a chemical element with atomic number 1. It has an atomic weight of 1.00794 u (or Da) (1.007825 u for the isotope 1H containing no neutrons). It is the lightest and most abundant element, making up ∼75 % of the Universe’s detectable mass and ∼90 % of its detectable atoms.
Main sequence stars are mostly composed of ionized hydrogen in plasmas, which plays a vital role in powering stars through proton-proton reactions and the CNO nuclear fusion cycle. Hydrogen has three naturally occurring isotopes: 1H, 2H, and 3H, which all contain 1 proton and 0, 1, or 2 neutrons, respectively. While hydrogen gas is rare in the Earth’s atmosphere (∼1 ppm by volume) because it escapes from the Earth’s gravity field more easily than heavier gases, it is the third most abundant element on the Earth’s surface, mainly as a component of water.
Its most common isotope is 1H (protium), which contains a single proton. In ionic compounds, hydrogen can take a...