The lightest chemical element on the periodic table is hydrogen. A neutral hydrogen-1 atom has one proton and one electron, giving hydrogen the lowest atomic mass of all elements.
Hydrogen is represented by H and has atomic number 1. Under ordinary conditions it is a colorless, odorless gas whose molecules usually contain two hydrogen atoms, written H₂. It is the most abundant element in the universe by mass and a major component of stars.
In 1766, Henry Cavendish identified hydrogen as a distinct substance. Antoine Lavoisier later gave it the name hydrogen, derived from Greek roots associated with water formation, because hydrogen burns with oxygen to produce water.
Hydrogen is not the same as helium, even though both are very light gases. Hydrogen is highly flammable, while helium is a chemically unreactive noble gas. Hydrogen also occurs in water, hydrocarbons, acids, and many biological molecules.