A colorless, odorless, tasteless, inert gaseous chemical element. Helium’s symbol is He, its atomic number is 2, and its atomic weight is 4.0026. Because helium will not combine with other chemical elements, it is used to force oxygen away from the molten metal in certain types of arc welding, and to fill some types of electron tubes to prevent oxygen from contacting the hot filaments.

Because of its lightness and because it will not burn, helium is used to inflate lighter-than-air aircraft.