Helium is the second lightest element after hydrogen. This transparent gas was discovered in 1868 by Sir William Ramsay. Today, we know that a quarter of all atoms in the Universe are helium. It is one of the main gases in the atmospheres of the giant gas planets, such as Saturn. However, being so light, helium is very rare to find on Earth, as it escapes from our atmosphere into space. Thus, it was not until 1895 that chemists managed to collect a sample of helium gas from uraninite, a radioactive uranium ore. Today, helium is collected from subway deposits or is found mixed in natural gas and oil.
Unlike hydrogen, which is very reactive, helium is a noble gas and does not react at all. This property makes it a safe gas to use in objects such as balloons and aircraft. To liquefy it, it must be cooled to a temperature of -269 °C (-269 °F). Liquid helium is used for freezing at low temperatures, including the powerful magnets used to make magnetic levitation trains float on special tracks. Nuclear magnetic resonance scanners also use liquid helium for cooling.
Summary of properties (He)
|Discoverer (year)||Ramsey, Sir William & Cleve, Per Teodor (1895)|
|Melting point (ºC)||-272|
|Boiling point (ºC)||-269|
|Abundance in earth's crust (ppm)||0.008|
|Isotope (abundance)||3He (0.000134), 4He (99.999866)|
|Van der Waals radius (pm)||140|
|Covalent radius (pm)||37|
|Vaporization enthalpy (kJ/mol)||0.08|
|Enthalpy of fusion (kJ/mol)||-|
|Specific heat capacity (J/g·K) at 25ºC and 1 atm||5.19|
|Thermal conductivity (W/cm·K) at 25ºC and 1 atm||0.002|
|Electron affinity (eV)|
|1st Ionization potential (eV)||24.5874|