Hydrogen is a chemical element at the start of the periodic table. It has the symbol H and atomic number 1. It also has a standard atomic weight of 1.008, which makes it the lightest element in the periodic table.
Hydrogen is the most common chemical element in the Universe, making up 75% of all normal (baryonic) matter (by mass). Most stars are made of mostly hydrogen. Hydrogen's most common isotope has one proton with one electron orbiting around it.