Helium

Under standard temperature and pressure helium exists only as a monatomic gas. Helium condenses only under very extreme conditions.

It has the lowest melting point of any element and is the only liquid that can't be solidified by lowering its temperature; remaining liquid all the way to absolute zero at standard pressure (it can only be solidified by increasing the pressure). In fact, the critical temperature, above which there is no difference between the liquid and gaseous phases, is only 5.19 K. Solid He-3 and He-4 are unique in that by applying pressure a researcher can change their volumes by more than 30%. The specific heat capacity of helium gas is very high and helium vapor is very dense, expanding rapidly when it is warmed to room temperature.

Solid helium only exists at great pressures, around 100 MPa at 15 K, and at roughly this temperature helium undergoes a transition between high temperature and low temperature forms, in which the atoms have cubic and hexagonal close packings, respectively. At a fraction of the temperature and pressure a third form occurs where the atoms have a body-centered cubic arrangement. All these arrangements are fairly similar in energy and density, and the reasons for the changes have to do with the details of how the atoms interact.

It is often used as a lifting gas in lighter-than-air vessels which in turn are used for advertising, atmospheric research, military reconnaissance and as a novelty. In addition, helium has 92.64% of the lifting power of hydrogen but is not flammable and is therefore considered safer. Other uses;

• Helium-oxygen atmospheres are used in high-pressure breathing work such as deep diving suits or submersibles because helium is inert, less soluble in blood than nitrogen, and diffuses 2.5 times faster than nitrogen. This reduces the time required for degassing on decompression, eliminates the danger of nitrogen narcosis, and is less likely to collect as bubbles in joints.
• It has the lowest melting and boiling points of any element which makes liquid helium an ideal coolant for many extremely low-temperature applications such as superconducting magnets and cryogenic research where temperatures close to absolute zero are needed.
• Helium is also used as an inert carrier gas, such as in gas chromatography.
• The fusion of hydrogen into helium provides the energy needed for the hydrogen bomb.
• It is also used for; pressuring liquid fuel rockets, as an inert gas shield for arc welding, as a protective gas for growing silicon and germanium crystals, as a cooling agent for nuclear reactors, and as a gas for supersonic wind tunnels.

Helium (Greek helios meaning "the sun") was discovered by Frenchman Pierre Janssen and Englishman Norman Lockyer working independently of each other in 1868. Both men were studying light from the sun during a solar eclipse that year and spectroscopically found an emission line of a previously unknown element. Eduard Frankland confirmed Janssen's findings and also proposed that the element should named after Helios, the Greek god of the sun, with the added suffix -ium because the new element was expected to be a metal. It was isolated by Sir William Ramsay in 1895, from clevite and conclusively found to not be a metal, but the name was unchanged. Swedish chemists Nils Langlet and Per Theodor Cleve, working independently of Ramsay, also were able to isolate helium from clevite at about the same time.

In 1907 Ernest Rutherford and Thomas Royds were able to show that alpha particles are helium nuclei. In 1908 Dutch physicist Heike Kamerlingh Onnes produced the first liquid helium by cooling the gas to 0.9°K, a feat that earned him a Nobel Prize. In 1926 his student Willem Hendrik Keesom was the first person to solidify helium.

Helium is the second most abundant element in the universe after hydrogen and forms about 20 percent of the matter in stars. It is also plays an important role in both the proton-proton reaction and the carbon cycle in stars which accounts for much of their energy. The abundance of helium is far too large to be explained by production by stars, but is consistent with the big bang model, and the vast majority of helium in the universe is believed to have been formed in the first three minutes of the universe.

This element is also present in earth's atmosphere at about 1 part in 200,000 and is found as a decay product in various radioactive minerals. Specifically, it is found in minerals of uranium and thorium, such as clevites, pitchblende, carnotite, monazite and beryl; it is produced from these elements by radioactive decay in the form of alpha particles. It is also found in some mineral waters (1 part helium per thousand water in some Iceland springs), in volcanic gases, and in certain natural gas deposits in the United States (from which most of the commercial helium on Earth is derived). Helium can be synthesized by bombardment of lithium or boron by high-velocity protons.

Helium is the first of the noble gases and is chemically unreactive for practical purposes but under the influence of electric glow discharge or electron bombardment helium does form compounds with tungsten, iodine, fluorine, sulfur and phosphorus.

The most common isotope of helium is He-4, where the nucleus has two protons and two neutrons. This is an unusually stable nuclear arrangement since it has a magic number of nucleons, that is, a number where they are arranged into complete shells. Many heavier nuclei decay by the emission of He-4 nuclei, a process called alpha decay, and helium nuclei are thus called alpha particles. Most of the helium on earth is generated by this process. Helium has a second isotope, helium-3, where the nucleus only has a single neutron, as well as several heavier isotopes that are radioactive. Helium-3 is virtually unknown on the Earth's surface, as the internal sources of helium only produce the He-4 isotope as alpha particles and atmospheric helium escapes into space over relatively short geological timescales.

Liquid helium (He-4) is found in two forms: He-4 I and He-4 II, which share a sharp transition point at 2.1768 K at its vapor pressure. He-4 I (above this point) is a normal liquid, but He-4-II (below this temperature) is unlike any other known substance.

As it is cooled past 2.1768 K at its vapor pressure, the so-called lambda point, it becomes a superfluid known as liquid Helium II (as opposed to "normal" liquid Helium I) which has many unusual characteristics due to quantum effects; it was one of the first observed examples of quantum effects operating on a macroscopic scale. This transition takes place at much lower temperatures in Helium-3 than it does in Helium-4, as the effect relies on condensation of bosons but the nuclei of the former are fermions, which can't condense individually but must do so in bosonic pairs. Since the transformation is one of higher order, without latent heat at the lambda point, the two liquid forms never coexist.

Helium II has zero viscosity and has a heat conductivity much higher than any other substance. Furthermore, helium II exhibits a thermomechanical (fountain) effect; if two vessels containing helium II are connected by a narrow capillary and one of the two is heated a flow of helium toward the heated vessel will occur. Conversely, in the mechanocaloric effect, a forced flow of helium II through a capillary will result in cooling of the helium II leaving the capillary. Pulses of heat introduced into helium II will propagate through the liquid in the same manner as the density pulses of sound, a phenomenon which has been dubbed "second sound." Solid surfaces in contact with helium II are covered with a film 50 to 100 atoms thick, along which frictionless flow of the liquid can occur; as a result it is impossible to contain helium II in an open vessel without it flowing out over the edge. Mass transport through the helium II film takes place at a constant rate which only depends on temperature. Finally, a mass of helium II will not rotate as a unit; instead, attempts to set it rotating will induce small frictionless vortices throughout the liquid.

Containers filled with gaseous helium at 5 to 10 K should be stored as if they contained liquid helium due to the large increase in pressure that results from warming the gas to room temperature.

Further Reading