The second smallest and second most abundant element in the universe1. Colorless, odorless, tasteless (I assume), nonreactive, consisting of only two protons, two neutrons, and two electrons in its normal state, the lightest of the noble gasses and the first discovered.

In the unimaginably hot interiors of midsize stars, hydrogen undergoes fusion, releasing tremendous energy and yielding helium; consequentially, helium now accounts for about 26% of the universe's stellar mass. The mass of individual helium molecules is so small, however, that the gravity of planet-sized bodies cannot hold them permanently: Earthly helium rises, baloonlike, to the top of the atmosphere (it is a chief component of the rarefied exosphere) and then continues to rise, escaping Earth's main gravitational pull. Its concentration back at sea level is vanishingly tiny, about 5 parts per million.2

Unsurprisingly, then, helium was undiscovered until the 19th century. In 1868, during a solar eclipse, the French astronomer Pierre Janssen pointed a spectrometer at the suddenly visible corona and found a bright yellow line that he took to represent sodium. Later that year, Joseph Lockyer, a British astronomer, outfitted his telescope with a spectrometer and observed that the yellow line did not correspond with the sodium's two known lines. Concluding that he had discovered an element that existed only in the sun, he named it using helios, "Sun" in Greek. In 1895, British chemist William Ramsay analyzed the gas produced in the heating of cleveite, a uranium-containing mineral, and found helium's characteristic yellow line, the first evidence of helium on Earth.3

In the years since, the properties of helium have been revealed. It has, for example, another stable isotope in addition to its two-neutron variety: helium-3, which has only one neutron and is produced in the reverse beta decay of hydrogen-3 (tritium). Tritium being rare, helium-3 is present in concentrations of only about 7 parts per trillion.

The thermal properties of helium are especially interesting. A member of the noble gasses, its molecules are drawn to each other only by dispersion forces, and with an atomic structure so small, even those are almost nonexistent. Consequentially, helium has an extraordinarily low boiling point: 4.22 degrees Kelvin.4 Every other element is a solid below 14.01ºK, at which point hydrogen becomes a liquid. This has made helium a popular coolant for use in cryogenics, and an invaluable tool in superconductivity research.

In addition, helium, uniquely, has a second liquid state5 that begins below 2.8ºK, at which temperature its viscosity drops to almost zero and its thermal conductivity becomes 1,000 times greater than that of copper. Superfluid helium, as it is called, can also flow through capillaries too small for any other element, or, as a thin film apparently oblivious to the law of gravity, flow up and over the rim of its container.

Of course, most people are familiar with more everyday properties of helium. Because of its lower density, sound waves travel more quickly through it than through air, so the frequencies amplified the most by a container with one open end (the particular wavelengths of sound that fit best, called the resonant frequencies) must become higher. This has the well-known effect of raising the pitch of voice of anyone who inhales the substance.6

Perhaps the most famous property of helium is its buoyancy; children marvel at balloons that stretch a string taut and float infinitely upward when released.7 Scientists have found uses for balloons, too - and, after several airship disasters, all of these balloons use helium, rather than the readily combustible hydrogen.

Helium has replaced combustible gasses in heavier-than-air craft, as well. During the early years of the space program, NASA tested the integrity of rockets and spacecraft by increasing the interior pressure to several atmospheres. After a single spark in the dense oxygen atmosphere of an Apollo rocket resulted in a fire that killed the five suited astronauts who were on board testing other systems, pressure tests have been conducted using helium, in spaceships empty of people.

The deep ocean, another high-pressure environment, has presented its own set of difficulties. SCUBA divers use air tanks that pressurize to the water pressure in which they swim, but nitrogen, a chief component of air, has a tendency at high pressures to dissolve in the bloodstream, and is psychoactive. Consequentially, in SCUBA tank air nitrogen is replaced with helium.

  1. After Hydrogen, in both cases.
  2. The natural gas found in the United States, and to a lesser extent in Canada, South Africa, and the Sahara Desert, contains up to 7.6% helium, however. Cooling the natural gas until all elements but helium have liquefied yields gas that is 98.2% pure; using charcoal to absorb the other elements as gasses yields 99.995% purity.
  3. The Earth doesn't undergo fusion, of course (at least, not under normal circumstances). The helium molecules present here are the detritus of an opposite nuclear process, radioactive decay - which nicely explains their presence near uranium, as William Ramsay and Frederick Soddey discovered in 1903. Alpha particles can be considered doubly reduced helium atoms, clumps of two protons and two neutrons, with a positive charge that will quickly attract electrons to form standard helium.
  4. Helium is the only element without a solid phase at 1 atmosphere; at 25 atmospheres, its melting point is about 1ºK.
  5. Helium-3 has a third liquid state, as well, which is also superfluid.
  6. Inhaling helium can also cause oxygen deprivation, so be careful.
  7. The balloons themselves will pop, of course, when the difference in pressure between their interiors and exteriors becomes greater than the strength of their plastic, but the helium will diffuse and continue upward.


I wrote this in 2000, so I'm sure some of these no longer go anywhere

node your homework