Helium

Notable characteristics

Gas and plasma phases

Helium is a colorless, odorless, and non-toxic gas. It is the least reactive member of group 18 (the noble gases) of the periodic table and therefore virtually inert. Under standard temperature and pressure helium behaves very much like an ideal gas. Under virtually all conditions helium is monatomic. It has a thermal conductivity that is greater than any gas except hydrogen and its specific heat is unusually high. Helium is also less water soluble than any other gas known and its diffusion rate through solids is three times that of air and around 65% that of hydrogen. Helium's index of refraction is closer to unity than any other gas. This gas has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson inversion temperature (of about 40 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling.

Related Topics:
Noble gas - Standard temperature and pressure - Ideal gas - Thermal conductivity - Hydrogen - Specific heat - Soluble - Diffusion - Solid - Index of refraction - Joule-Thomson coefficient - Joule-Thomson inversion temperature - K

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Helium is chemically unreactive under all normal conditions due to its valence of zero. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He2++, HeH+, and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces . Theoretically, other compounds, like helium fluorohydride (HHeF), may also be possible.

Related Topics:
Valence - Ion - Energy level - Electrical - Voltage - Ionization potential - Compound - Tungsten - Iodine - Fluorine - Sulfur - Phosphorus - Electric glow discharge - Plasma - Band system

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Throughout the Universe, helium is mostly found in the plasma state whose properties are quite different to molecular helium. As a plasma, helium's electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields, for example, in the solar wind together with ionized hydrogen, they interact with the Earth's magnetosphere giving rising to Birkeland currents and the aurora.

Related Topics:
Plasma - Solar wind - Magnetosphere - Birkeland current - Aurora

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Solid and liquid phases

Helium solidifies only under great pressure. The resulting colorless almost invisible solid is highly compressible; applying pressure in the laboratory can decrease its volume by more than 30%. With a bulk modulus on the order of 5×107 Pa http://www3.interscience.wiley.com/cgi-bin/abstract/105558571/ABSTRACT it is 50 times more compressible than water. Unlike any other element, helium will fail to solidify and remain a liquid down to absolute zero at normal pressures. Solid helium requires a temperature of 1–1.5 K and about 26 standard atmospheres (2.6 MPa) of pressure. It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure.

Related Topics:
Solid - Compressible - Bulk modulus - Pa - Absolute zero - Refractive index - Melting point - Crystal

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Helium I state

Below its boiling point of 4.21 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when heat is added to it. It also contracts when its temperature is lowered until it reaches the lambda point, when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1 K is reached; at which point expansion completely stops and helium I starts to contract again.

Related Topics:
Boiling point - Kelvin - Lambda point - Isotope - Liquid

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Helium I has a gas-like index of refraction of 1.026 which makes its surface so hard to see that floats of Styrofoam are often used to show where the surface is. This colorless liquid has a very low viscosity and a density 1/8th that of water, which is only 1/4th the value expected from classical physics. Quantum mechanics is needed to explain this property and thus both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This is probably due to its boiling point being so close to absolute zero, which prevents random molecular motion (heat) from masking the atomic properties.

Related Topics:
Index of refraction - Styrofoam - Viscosity - Density - Water - Classical physics - Quantum mechanics - Heat

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Helium II state

Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; heat input instead causes evaporation of the liquid directly to gas. The isotope helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope helium-3.

Related Topics:
Thermal conductivity - Evaporation

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Helium II is a superfluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10-7 to 10-8 m width it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for Helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.

Related Topics:
Superfluid - Viscosity - Ground state

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, B. V. Rollin. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate.

Related Topics:
Gravity - Nm - Rollin film - B. V. Rollin

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air. So when heat is introduced, it will move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called second sound.

Related Topics:
Copper - Valence band - Flow of heat - Equation - Wave equation - Sound

~ ~ ~ ~ ~ ~ ~ ~ ~ ~