Helium is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert monatomic GAS. Helium is the second lightest and second most abundant element in the universe, making up about 24% of the mass of the universe. It is present in the atmosphere of Earth at a concentration of 5.2 ppm by volume. Helium is used in cryogenics, welding, and as a lifting GAS in balloons and airships. The GAS form of helium has the three-letter abbreviation GAS.