• Much the most common element in the universe, hydrogen accounts for most known matter and has given rise to all the rest by means of fusion. A hydrogen atom is the simplest imaginable, consisting of a proton (containing almost all the mass) and an electron in orbit around it. Hydrogen at reasonably low temperatures and where it is in sufficient concentration will form a stable molecule of two atoms. Hydrogen in deep space can be mapped by radio emissions at a wavelength of 21cm. This is the frequency of the energy absorbed or emitted when the electron shifts orbit from moving in the same direction as the proton’s own rotation to moving in the opposite direction, which involves slightly less energy. Hydrogen has two isotopes, deuterium and tritium.
  • A chemical element whose atomic number is 1. It is the lightest element, and is a colorless diatomic gas. Hydrogen has 3 isotopes, each with its own name: protium, deuterium, and tritium, with tritium being radioactive. Hydrogen is present in countless chemical compounds, and is the most abundant element in the known universe. It has various applications, including its use as a fuel, in welding, low-pressure research, low-temperature research, and in the production of high-purity metals.
  • chemical symbol H

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