Helium (He) is the 2nd element of the periodic table as you know. It is a noble gas. Apart from being a noble gas, it shows some very interesting properties; that is why it has won the top place in Zwitterion's Favorites List. I want to give some information about this amazing element here. If you are interested, mail me at: e105283@orca.cc.metu.edu.tr

Helium is a colorless, odorless gas at room temperature. Its name comes from the Greek word "Helios" (Sun); since a spectrum of the Sun taken by Pierre Janssen during an eclipse in 1868 became the first evidence of its existence. It was first isolated from clevite (an uranium mineral) in 1895.

Helium has two natural isotopes: He-4 mostly and He-3 in trace amount (0.00013%). He-5, He-6 and He-8 have been synthesized. It has a very low boiling point: 4.22K (-268.93 degrees C). Moreover it cannot be solidified at atmospheric pressure (even at 0K). It solidifies at 0.95K temperature and 26atm pressure.

Here I would like to mention the first property that gives Helium its uniqueness: When liquid helium (named helium 1) is cooled below 2.174K (at about 0.05 atm), known as lambda point, a new state of matter is obtained. This new state is called helium 2, and shows extraordinary properties in contrast with helium 1 which shows normal liquid characteristics. Helium 2 is a superfluid - something between liquid and gas. It is a very good thermal conducter (approximately 600 times better than copper at room temperature). It expands when cooled. It has a very low viscosity (much lower than that of hydrogen gas [about 1/1000]). And may be the most interesting property: If put inside a vessel, it climbs up the walls of the vessel and over the top.

This unusual property is explained through the properties of Bose-Einstein particles (or bosons for short). These particles can be put into the same quantum state. Einstein even showed how the nature of Bose partiles can force them into the same state without there being interactions present. This phenomenon is seen in lasers (for light) and in BEC (Bose-Einstein Condensate) and in helium 2. Normally substances that we observe in daily life consist of Fermi-Dirac particles (or simply fermions). In crude terms, Fermions (good examples to which are electrons) can be considered not to want their de Broglie waves to overlap. For more information you can look at the 269th volume of the magazine "Science".

Another property that gives helium its importance is its abundance in the universe: It is only the second after hydrogen. Its presence in the Earth's atmosphere is only about 1/200,000.

Other Info:
Atomic weight: 4.00260
First ionization potential: 24.587 V
Atomic Radius: 0.49 Angstrom
Covalent Radius: 0.93 Angstrom
Critical Temperature: 5.25 K
Boiling Point for He-3: 3.2 K
Crystal Structure: Hexagonal
Heat of Vaporization: 0.0845 kJ/mol

Bibliography:
1- Science, Vol. 269, 14 July 1995, pages 152,182
2- The 1995 Grolier Multimedia Encylodia
3- Chart of Nuclides, Knolls Atomic Power Laboratory, Ninth Edition, July 1966
4- Table of Periodic Properties of The Elements, Sargent-Welch Scientific Company, Catalog No. S-188806

Some related links:
Superfluids and Helium Gyroscope
Webelements - Periodic Table on WWW

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