Helium (from Greek: ἥλιος, romanized: helios, lit. 'sun') is a chemical element; it has the symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is the lowest among all the elements, and it does not have a melting point at standard pressures. It is the second-lightest and second-most abundant element in the observable universe, after hydrogen. It is present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this in both the Sun and Jupiter, because of the very high nuclear binding energy (per nucleon) of helium-4 concerning the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of both nuclear fusion and radioactive decay. The most common isotope of helium in the universe is helium-4, the vast majority of which was formed during the Big Bang. Large amounts of new helium are created by nuclear fusion of hydrogen in stars.
Helium was first detected as an unknown, yellow spectral
line signature in sunlight during a solar eclipse in 1868 by Georges Rayet,
Captain C. T. Haig, Norman R. Pogson, and Lieutenant John Herschel, and was
subsequently confirmed by French astronomer Jules Janssen. Janssen is often
jointly credited with detecting the element, along with Norman Lockyer. Janssen
recorded the helium spectral line during the solar eclipse of 1868, while
Lockyer observed it from Britain. However, only Lockyer proposed that the line
was due to a new element, which he named after the Sun. The formal discovery of
the element was made in 1895 by chemists Sir William Ramsay, Per Teodor Cleve,
and Nils Abraham Langlet, who found helium emanating from the uranium ore
cleveite, which is now not regarded as a separate mineral species, but as a
variety of uraninite. In 1903, large reserves of helium were found in natural
gas fields in parts of the United States, by far the largest supplier of the
gas today.
Liquid helium is used in cryogenics (its largest single use,
consuming about a quarter of production) and in the cooling of superconducting
magnets, with its main commercial application in MRI scanners. Helium's other
industrial uses—as a pressurizing and purge gas, as a protective atmosphere for
arc welding, and in processes such as growing crystals to make silicon
wafers—account for half of the gas produced. A small but well-known use is as a
lifting gas in balloons and airships. As with any gas whose density differs
from that of air, inhaling a small volume of helium temporarily changes the
timbre and quality of the human voice. In scientific research, the behavior of
the two fluid phases of helium-4 (helium I and helium II) is important to
researchers studying quantum mechanics (in particular, the property of
superfluidity) and to those looking at the phenomena, such as
superconductivity, produced in matter near absolute zero.
On Earth, it is relatively rare, 5.2 ppm by volume in the
atmosphere. Most terrestrial helium present today is created by the natural
radioactive decay of heavy radioactive elements (thorium and uranium, although
there are other examples), as the alpha particles emitted by such decays consist
of helium-4 nuclei. This radiogenic helium is trapped with natural gas in
concentrations as great as 7% by volume, from which it is extracted
commercially by a low-temperature separation process called fractional
distillation. Terrestrial helium is a non-renewable resource because once
released into the atmosphere, it promptly escapes into space. Its supply is
thought to be rapidly diminishing. However, some studies suggest that helium
produced deep in the Earth by radioactive decay can collect in natural gas
reserves in larger-than-expected quantities, in some cases having been released
by volcanic activity.
History
Scientific discoveries
The first evidence of helium was observed on August 18,
1868, as a bright yellow line with a wavelength of 587.49 nanometers in the
spectrum of the chromosphere of the Sun. The line was detected by French
astronomer Jules Janssen during a total solar eclipse in Guntur, India. This
line was initially assumed to be sodium. On October 20 of the same year,
English astronomer Norman Lockyer observed a yellow line in the solar spectrum,
which he named the D3 because it was near the known D1 and D2 Fraunhofer lines
of sodium. He concluded that it was caused by an element in the Sun unknown on
Earth. Lockyer named the element with the Greek word for the Sun, ἥλιος (helios). It is sometimes
said that English chemist Edward Frankland was also involved in the naming, but
this is unlikely, as he doubted the existence of this new element. The ending "-ium"
is unusual, as it normally applies only to metallic elements; probably Lockyer,
being an astronomer, was unaware of the chemical conventions.
Spectral lines of helium
In 1881, Italian physicist Luigi Palmieri detected helium on
Earth for the first time through its D3 spectral line, when he analyzed a
material that had been sublimated during a recent eruption of Mount Vesuvius.
On March 26, 1895, Scottish chemist Sir William Ramsay
isolated helium on Earth by treating the mineral cleveite (a variety of
uraninite with at least 10% rare-earth elements) with mineral acids. Ramsay was
looking for argon, but after separating nitrogen and oxygen from the gas
liberated by sulfuric acid, he noticed a bright yellow line that matched the D3
line observed in the spectrum of the Sun. These samples were identified as
helium by Lockyer and British physicist William Crookes. It was independently
isolated from cleveite in the same year by chemists Per Teodor Cleve and
Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to
accurately determine its atomic weight. Helium was also isolated by American
geochemist William Francis Hillebrand before Ramsay's discovery, when he
noticed unusual spectral lines while testing a sample of the mineral uraninite.
Hillebrand, however, attributed the lines to nitrogen. His letter of
congratulations to Ramsay offers an interesting case of discovery, and
near-discovery, in science.
In 1907, Ernest Rutherford and Thomas Royds demonstrated
that alpha particles are helium nuclei by allowing the particles to penetrate
the thin glass wall of an evacuated tube, then creating a discharge in the
tube, to study the spectrum of the new gas inside. In 1908, helium was first
liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less
than 5 K (−268.15 °C; −450.67 °F). He tried to solidify it by further reducing
the temperature, but failed because helium does not solidify at atmospheric
pressure. Onnes' student Willem Hendrik Keesom was eventually able to solidify
1cm³3 of helium in 1926 by applying additional external pressure.
In 1913, Niels Bohr published his "trilogy" on
atomic structure that included a reconsideration of the Pickering–Fowler series
as central evidence in support of his model of the atom. This series is named
for Edward Charles Pickering, who in 1896 published observations of previously
unknown lines in the spectrum of the star ζ Puppis (these are now known to
occur with Wolf–Rayet and other hot stars). Pickering attributed the
observation (lines at 4551, 5411, and 10123 Å) to a new form of hydrogen with
half-integer transition levels. In 1912, Alfred Fowler managed to produce
similar lines from a hydrogen-helium mixture and supported Pickering's
conclusion as to their origin. Bohr's model does not allow for half-integer
transitions (nor does quantum mechanics), and Bohr concluded that Pickering and
Fowler were wrong and instead assigned these spectral lines to ionized helium,
He+. Fowler was initially skeptical but was ultimately convinced that Bohr was
correct, and by 1915, "spectroscopists had transferred [the
Pickering–Fowler series] definitively [from hydrogen] to helium."
Bohr's theoretical work on the Pickering series had demonstrated the need for "a
re-examination of problems that seemed already to have been solved within
classical theories" and provided important confirmation for his atomic
theory.
In 1938, Russian physicist Pyotr Leonidovich Kapitsa
discovered that helium-4 has almost no viscosity at temperatures near absolute
zero, a phenomenon now called superfluidity. This phenomenon is related to
Bose–Einstein condensation. In 1972, the same phenomenon was observed in
helium-3, but at temperatures much closer to absolute zero, by American
physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson. The phenomenon
in helium-3 is thought to be related to the pairing of helium-3 fermions to make
bosons, in analogy to Cooper pairs of electrons producing superconductivity.
In 1961, Vignos and Fairbank reported the existence of a
different phase of solid helium-4, designated the gamma-phase. It exists for a
narrow range of pressure between 1.45 and 1.78 K.
Extraction and use
After an oil drilling operation in 1903 in Dexter, Kansas
produced a gas geyser that would not burn, Kansas state geologist Erasmus
Haworth collected samples of the escaping gas and took them back to the
University of Kansas at Lawrence where, with the help of chemists Hamilton Cady
and David McFarland, he discovered that the gas consisted of, by volume, 72%
nitrogen, 15% methane (a combustible percentage only with sufficient oxygen),
1% hydrogen, and 12% an unidentifiable gas. With further analysis, Cady and
McFarland discovered that 1.84% of the gas sample was helium. This showed that
despite its overall rarity on Earth, helium was concentrated in large
quantities under the American Great Plains, available for extraction as a
byproduct of natural gas.
Following a suggestion by Sir Richard Threlfall, the United
States Navy sponsored three small experimental helium plants during World War
I. The goal was to supply barrage balloons with the non-flammable,
lighter-than-air gas. A total of 5,700 m3 (200,000 cu ft) of 92% helium was
produced in the program, even though less than a cubic meter of the gas had
previously been obtained. Some of this gas was used in the world's first
helium-filled airship, the U.S. Navy's C-class blimp C-7, which flew its maiden
voyage from Hampton Roads, Virginia, to Bolling Field in Washington, D.C., on
December 1, 1921, nearly two years before the Navy's first rigid helium-filled
airship, the Naval Aircraft Factory-built USS Shenandoah, flew in September
1923.
Although the extraction process using low-temperature gas
liquefaction was not developed in time to be significant during World War I,
production continued. Helium was primarily used as a lifting gas in
lighter-than-air craft. During World War II, the demand for helium increased for lifting gas and for shielded arc welding. The helium mass spectrometer was
also vital in the atomic bomb Manhattan Project.
The government of the United States set up the National
Helium Reserve in 1925 at Amarillo, Texas, to supply military airships in time
of war and commercial airships in peacetime. Because of the Helium Act of 1925,
which banned the export of scarce helium on which the US then had a production
monopoly, together with the prohibitive cost of the gas, German Zeppelins were
forced to use hydrogen as lifting gas, which would gain infamy in the
Hindenburg disaster. The helium market after World War II was depressed, but
the reserve was expanded in the 1950s to ensure a supply of liquid helium as a
coolant to create oxygen/hydrogen rocket fuel (among other uses) during the
Space Race and Cold War. Helium use in the United States in 1965 was more than
eight times the peak wartime consumption.
After the Helium Act Amendments of 1960 (Public Law 86–777),
the U.S. Bureau of Mines arranged for five private plants to recover helium
from natural gas. For this helium conservation program, the Bureau built a
425-mile (684 km) pipeline from Bushton, Kansas, to connect those plants with
the government's partially depleted Cliffside gas field near Amarillo, Texas.
This helium-nitrogen mixture was injected and stored in the Cliffside gas field
until needed, at which time it was further purified.
By 1995, a billion cubic meters of the gas had been
collected, and the reserve was US$1.4 billion in debt, prompting the Congress
of the United States in 1996 to discontinue the reserve. The resulting Helium
Privatization Act of 1996 (Public Law 104–273) directed the United States
Department of the Interior to empty the reserve, with sales starting by 2005.
Helium produced between 1930 and 1945 was about 98.3% pure
(2% nitrogen), which was adequate for airships. In 1945, a small amount of
99.9% helium was produced for welding use. By 1949, commercial quantities of
Grade A 99.95% helium were available.
For many years, the United States produced more than 90% of
commercially usable helium in the world, while extraction plants in Canada,
Poland, Russia, and other nations produced the remainder. In the mid-1990s, a
new plant in Arzew, Algeria, producing 17 million cubic meters (600 million
cubic feet) began operation, with enough production to cover all of Europe's
demand. Meanwhile, by 2000, the consumption of helium within the U.S. had risen
to more than 15 million kg per year. In 2004–2006, additional plants in Ras
Laffan, Qatar, and Skikda, Algeria, were built. Algeria quickly became the
second leading producer of helium. Throughout this time, both helium
consumption and the costs of producing helium increased. From 2002 to 2007,
helium prices doubled.
As of 2012, the United States National Helium Reserve
accounted for 30 percent of the world's helium. The reserve was expected to run
out of helium in 2018. Despite that, a proposed bill in the United States
Senate would allow the reserve to continue to sell the gas. Other large
reserves were in the Hugoton in Kansas, United States, and nearby gas fields of
Kansas and the panhandles of Texas and Oklahoma. New helium plants were
scheduled to open in 2012 in Qatar, Russia, and the US state of Wyoming, but
they were not expected to ease the shortage.
In 2013, Qatar started up the world's largest helium unit,
although the 2017 Qatar diplomatic crisis severely affected helium production
there. 2014 was widely acknowledged to be a year of oversupply in the helium
business, following years of renowned shortages. Nasdaq reported (2015) that
for Air Products, an international corporation that sells gases for industrial
use, helium volumes remain under economic pressure due to feedstock supply
constraints.
Characteristics
Atom
Picture of a diffuse gray sphere with grayscale density
decreasing from the center. Length scale about 1 Angstrom. An inset outlines
the structure of the core, with two red and two blue atoms at the length scale
of 1 femtometer.
The helium atom. Depicted are the nucleus (pink) and the
electron cloud distribution (black). The nucleus (upper right) in helium-4 is
in reality spherically symmetric and closely resembles the electron cloud,
although for more complicated nuclei this is not always the case.
In quantum mechanics
In the perspective of quantum mechanics, helium is the
second simplest atom to model, following the hydrogen atom. Helium is composed
of two electrons in atomic orbitals surrounding a nucleus containing two
protons and (usually) two neutrons. As in Newtonian mechanics, no system that
consists of more than two particles can be solved with an exact analytical
mathematical approach (see 3-body problem), and helium is no exception. Thus,
numerical mathematical methods are required, even to solve the system of one
nucleus and two electrons. Such computational chemistry methods have been used
to create a quantum mechanical picture of helium electron binding, which is
accurate to within < 2% of the correct value, in a few computational steps.
Such models show that each electron in helium partly screens the nucleus from
the other, so that the effective nuclear charge Zeff, which each electron sees,
is about 1.69 units, not the 2 charges of a classic "bare"
helium nucleus.
Related stability of the helium-4 nucleus and electron
shell
The nucleus of the helium-4 atom is identical to an alpha
particle. High-energy electron-scattering experiments show its charge to
decrease exponentially from a maximum at a central point, exactly as does the
charge density of helium's electron cloud. This symmetry reflects similar
underlying physics: the pair of neutrons and the pair of protons in helium's
nucleus obey the same quantum mechanical rules as do helium's pair of electrons
(although the nuclear particles are subject to a different nuclear binding
potential), so that all these fermions fully occupy 1s orbitals in pairs, none
of them possessing orbital angular momentum, and each canceling the other's
intrinsic spin. This arrangement is thus energetically extremely stable for all
these particles and has astrophysical implications. Namely, adding another
particle – proton, neutron, or alpha particle – would consume rather than
release energy; all systems with mass number 5, as well as beryllium-8
(comprising two alpha particles), are unbound.
For example, the stability and low energy of the electron
cloud state in helium account for the element's chemical inertness, and also
the lack of interaction of helium atoms with each other, producing the lowest
melting and boiling points of all the elements. In a similar way, the
particular energetic stability of the helium-4 nucleus, produced by similar
effects, accounts for the ease of helium-4 production in atomic reactions that
involve either heavy-particle emission or fusion. Some stable helium-3 (two
protons and one neutron) is produced in fusion reactions from hydrogen, though
its estimated abundance in the universe is about 10−5 relative to helium-4.
Binding energy per nucleon of common isotopes. The binding
energy per particle of helium-4 is significantly larger than all nearby
nuclides.
The unusual stability of the helium-4 nucleus is also
important cosmologically: it explains the fact that in the first few minutes
after the Big Bang, as the "soup" of free protons and neutrons
which had initially been created in about 6:1 ratio cooled to the point that
nuclear binding was possible, almost all first compound atomic nuclei to form
were helium-4 nuclei. Owing to the relatively tight binding of helium-4 nuclei,
its production consumed nearly all of the free neutrons in a few minutes,
before they could beta-decay, and thus, few neutrons were available to form
heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding
per nucleon is stronger than in any of these elements (see nucleogenesis and
binding energy), and thus, once helium had been formed, no energetic drive was
available to make elements 3, 4, and 5. It is barely energetically favorable
for helium to fuse into the next element with a lower energy per nucleon,
carbon. However, due to the short lifetime of the intermediate beryllium-8,
this process requires three helium nuclei striking each other nearly
simultaneously (see triple-alpha process).[90] There was thus no time for
significant carbon to be formed in the few minutes after the Big Bang, before
the early expanding universe cooled to the temperature and pressure point where
helium fusion to carbon was no longer possible. This left the early universe
with a very similar ratio of hydrogen/helium as is observed today (3 parts
hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the
universe trapped in helium-4.
All heavier elements (including those necessary for rocky
planets like the Earth, and for carbon-based or other life) have thus been
created since the Big Bang in stars which were hot enough to fuse helium
itself. All elements other than hydrogen and helium today account for only 2%
of the mass of atomic matter in the universe. Helium-4, by contrast, comprises
about 24% of the mass of the universe's ordinary matter—nearly all the ordinary
matter that is not hydrogen.
Gas and plasma phases
Helium is the second least reactive noble gas after neon,
and thus the second least reactive of all elements. It is chemically inert and
monatomic in all standard conditions. Because of helium's relatively low molar
(atomic) mass, its thermal conductivity, specific heat, and sound speed in the
gas phase are all greater than any other gas except hydrogen. For these reasons
and the small size of helium monatomic molecules, helium diffuses through
solids at a rate three times that of air and around 65% that of hydrogen.
Helium is the least water-soluble monatomic gas, and one of
the least water-soluble of any gas (CF4, SF6, and C4F8 have lower mole fraction
solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's
0.70797 x2/10−5), and helium's index of refraction is closer to unity than that
of any other gas. Helium 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 32 to 50 K at 1
atmosphere) does it cool upon free expansion. Once precooled below this
temperature, helium can be liquefied through expansion cooling.
Most extraterrestrial helium is plasma in stars, with
properties quite different from those of atomic helium. In a plasma, helium's
electrons are not bound to its nucleus, 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, the particles interact with
the Earth's magnetosphere, giving rise to Birkeland currents and the aurora.
Liquid phase
Liquefied helium. This helium is not only liquid, but has
been cooled to the point of superfluidity. The drop of liquid at the bottom of
the glass represents helium spontaneously escaping from the container over the
side, to empty the container. The energy to drive this process is supplied by
the potential energy of the falling helium.
Helium liquifies when cooled below 4.2 K at atmospheric
pressure. Unlike any other element, however, helium remains liquid down to a
temperature of absolute zero. This is a direct effect of quantum mechanics:
specifically, the zero-point energy of the system is too high to allow
freezing. Pressures above about 25 atmospheres are required to freeze it. There
are two liquid phases: Helium I is a conventional liquid, and Helium II, which
occurs at a lower temperature, is a superfluid.
Helium I
Below its boiling point of 4.22 K (−268.93 °C; −452.07 °F)
and above the lambda point of 2.1768 K (−270.9732 °C; −455.7518 °F), the
isotope helium-4 exists in a normal colorless liquid state, called helium I.
Like other cryogenic liquids, helium I boils when it is heated and contracts
when its temperature is lowered. Below the lambda point, however, helium does
not boil, and it expands as the temperature is lowered further.
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 of 0.145–0.125 g/mL (between about 0 and 4 K), which is only one-fourth
the value expected from classical physics. Quantum mechanics is needed to
explain this property, and thus both states of liquid helium (helium I and
helium II) are called quantum fluids, meaning they display atomic properties on
a macroscopic scale. This may be an effect of its boiling point being so close
to absolute zero, preventing random molecular motion (thermal energy) from
masking the atomic properties.
Helium II
Liquid helium below its lambda point (called helium II)
exhibits very unusual characteristics. Due to its high thermal conductivity,
when it boils, it does not bubble but rather evaporates directly from its
surface. Helium-3 also has a superfluid phase, but only at much lower
temperatures; as a result, less is known about the properties of the isotope.
Unlike ordinary liquids, helium II will creep along surfaces
to reach an equal level; after a short while, the levels in the two containers
will equalize. The Rollin film also covers the interior of the larger
container; if it were not sealed, the helium II would creep out and escape.
Helium II is a superfluid, a quantum mechanical state of
matter with strange properties. For example, when it flows through capillaries
as thin as 10 to 100 nm, it has no measurable viscosity. However, when
measurements were done between two moving discs, a viscosity comparable to that
of gaseous helium was observed. Existing 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.
In the fountain effect, a chamber is constructed that 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. 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 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 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. When heat is introduced, it moves at 20 meters per second
at 1.8 K through helium II as waves in a phenomenon known as second sound.
Helium II also exhibits a creeping effect. When a surface
extends past the level of helium II, the helium II moves along the surface,
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, Bernard 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. 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. Waves propagating across a rolling film are governed
by the same equation as gravity waves in shallow water, but rather than
gravity, the restoring force is the van der Waals force. These waves are known
as the third sound.
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