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 state
5 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.
-
After Hydrogen, in both cases.
-
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.
-
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.
-
Helium is the only element without a solid phase at 1 atmosphere; at 25 atmospheres, its melting point is about 1ºK.
-
Helium-3 has a third liquid state, as well, which is also superfluid.
-
Inhaling helium can also cause oxygen deprivation, so be careful.
-
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.
Sources:
http://www.britannica.com/bcom/eb/article/1/0,5716,1721+1+1713,00.html
http://www.webelements.com/webelements/elements/text/He/key.html
http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html
http://daily.stanford.org/Daily96-97/3-12-97/NEWS/NEWbug12.html
http://www.speclab.com/elements/helium.htm
I wrote this in 2000, so I'm sure some of these no longer go anywhere
node your homework