Helium Atomic Number: 2
Helium Symbol: He
Helium Atomic Weight: 4.002602(2)
Helium Discovery: Janssen, 1868, some sources say Sir William Ramsey, Nils Langet, P.T. Cleve 1895
Helium Electron Configuration: 1s2
Word Origin: Greek: helios, sun. Helium was first detected as a new spectral line during a solar eclipse.
Isotopes: 7 isotopes of helium are known.
Properties: Helium is a very light, inert, colorless gas. Helium has the lowest melting point of any element. It is the only liquid that cannot be solidified by lowering the temperature. It remains liquid down to absolute zero at ordinary pressures, but can be solidified by increasing the pressure. The specific heat of helium gas is unusually high. The density of helium vapor at the normal boiling point is also very high, with the vapor expanding greatly when heated to room temperature. Although helium normally has a valence of zero, it has a weak tendency to combine with certain other elements.
Uses: Helium is widely used in cryogenic research because its boiling point is near absolute zero. It is used in the study of superconductivity, as an inert gas shield for arc welding, as a protective gas in growing silicon and germanium crystals and producing titanium and zirconium, for pressuring liquid fuel rockets, for use in magnetic resonance imaging (MRI), as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels. A mixture of helium and oxygen is used as an artificial atmosphere for divers and others working under pressure. Helium is used for filling balloons and blimps.
Sources: Except for hydrogen, helium is the most abundant element in the universe. It is an important component in the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. Helium is extracted from natural gas. In fact, all natural gas contains at least trace quantities of helium. The fusion of hydrogen into helium is the sources of a hydrogen bomb's energy. Helium is a disintegration product of radioactive substances, so it is found in ores of uranium, radium, and other elements.
Element Classification: Noble Gas or Inert Gas
Usual Phase: gas
Density (g/cc): 0.1786 g/L (0 °C, 101.325 kPa)
Liquid Density (g/cc): 0.125 g/mL (at its boiling point)
Melting Point (°K): 0.95
Boiling Point (°K): 4.216
Critical Point: 5.19 K, 0.227 MPa
Atomic Volume (cc/mol): 31.8
Ionic Radius: 93
Specific Heat (@20°C J/g mol): 5.188
Heat of Fusion: 0.0138 kJ/mol
Evaporation Heat (kJ/mol): 0.08
First Ionizing Energy (kJ/mol): 2361.3
Lattice Structure: Hexagonal
Lattice Constant (Å): 3.570
Lattice C/A Ratio: 1.633
Crystal Structure: close-packed hexagonal
Magnetic Ordering: diamagnetic
CAS registry number: 7440-59-7
Sources: IUPAC (2009), Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952) International Atomic Energy Agency ENSDF database (Oct 2010)