 Hydrogen is a chemical element with chemical symbol H and atomic number 1 . With an atomic weight of 1 @.@ <unk> u , 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 1H ) , has one proton and no neutrons . 
