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High Quality Content by WIKIPEDIA articles! Hydrogen ( /EE haEdrEudE'E*n/ hy-drE(TM)-jin) is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of 1.00794 u (1.007825 u for Hydrogen-1), hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass.Stars in the main sequence are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth.