Hydrogen Hydrogen Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed "protium" (name rarely used, symbol H), has one proton and no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless,
History of the periodic table known as isotopes, from the Greek "" ("same place"). In 1914, a year before he was killed in action at Gallipoli, the English physicist Henry Moseley found a relationship between the X-ray wavelength of an element and its atomic number. He was then able to re-sequence the periodic table by nuclear charge, rather than by atomic weight. Before this discovery, atomic numbers were sequential numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers were in fact based upon experimental measurements. Using information about their X-ray wavelengths, Moseley placed argon (with an atomic number Z=18) before potassium