268 | The History and Use of Our Earth’s Chemical Elements AbundanceandSource
268 | The History and Use of Our Earth’s Chemical Elements AbundanceandSource
Argon is the 56th most abundant element on Earth. It is the most abundant of all the noble gases found in the atmosphere. In fact, the only source of argon is the atmosphere, where it is found at just under 1% of air by volume.
There are several methods of producing argon. The most common is by fractional distil- lation of liquid air. Argon is collected as a by-product of this large-scale commercial process. During fractional distillation, argon boils off at its own unique temperature. It is then col- lected and purified by passing it through charcoal to filter out helium and other gases, produc- ing significant amounts of argon.
History John William Strutt, commonly referred to as Lord Rayleigh (1842–1919), isolated a gas
that would not combine with oxygen and could not be identified. With the assistance of William Ramsay (1852–1916), Rayleigh separated the gas and named it “argon,” after the Greek word argos, which means “inert,” “inactive,” or “lazy.” Both were credited with the discovery of argon, which was identified by the then-new technology called spectroscopy. The spectroscope can identify each element, when heated, by the color and lines it forms on
a light spectrum. Each element’s spectrum is unique. Lord Rayleigh located a position on the periodic table where the atomic weight of argon matched the weight predicted for that vacant spot, and thus in 1894 he and Ramsay concluded that they had discovered a new element. This was the discovery of the first of the noble gases.
CommonUses Argon is used when an inert atmosphere is required. Individually, or as mixture with other
inert gases, it is used to fill electric light bulbs, fluorescent tubes, lasers, and so forth. By replacing oxygen in incandescent light bulbs, it prevents oxygen from corroding the bulb’s filament. It is also used as a nonoxidizing gas for welding and to decarbonize steel and as an inert atmosphere in which to grow semiconductor crystals.
ExamplesofCompounds As mentioned, it has recently been discovered that argon can only form fragile compounds
with a few other elements at extremely low temperatures. For example, under extreme cold, argon can combine with fluorine and hydrogen to form the fragile molecule HArF.
Hazards Argon is nontoxic, but as an asphyxiant gas, it can smother by replacing oxygen in the
lungs KRYPTON SYNBOL:Kr PERIOD:4 GROUP:18(VIIIA) ATOMICNO:36
ATOMICMASS:83.798amu VALENCE:0and2 OXIDATIONSTATE:0 NATURAL
STATE:Gas ORIGINOFNAME:Thename“krypton”isderivedfromtheGreekwordkryptos,meaning
“hidden.”
Guide to the Elements |
ISOTOPES:Thereareatotalof37isotopesofkrypton.Sixofthesearestable:Kr-78,Kr-80, Kr-82,Kr-83,Kr-84,andKr-86.TheisotopeKr-78hassuchalonghalf-life(0.9×10 +20 years)thatitisconsideredstableeventhoughitcontributesonly0.35%tothenatural kryptonintheEarth’satmosphere.Alltheothersareradioactive,man-madeby-products ofnuclearpowerplantsandradioactiveisotopeswithhalf-livesrangingfrom107nano- secondsto2.29×10 +15 years.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=8
s2,p6
Properties Krypton is a rather dense, tasteless, colorless, odorless gas. Its critical temperature is
between that of oxygen and carbon dioxide. It is extracted during fractional distillation of liquid oxygen at a temperature of about –63.8°C. At one time it was thought that krypton, as well as the other noble gases, were completely inert. However, in 1967 scientists were able to combine fluorine with krypton at low temperatures to form the compound krypton difluoride
(KrF 2 ). In this case krypton has a valence of 2. Krypton’s melting point is –156.6°C, its boiling point is –152.30°C, and its density is 0.003733g/cm 3 .
Characteristics Krypton is the fourth element in group 18 (VIIIA), which is also known as group 0 because
the elements is this group were thought to have a zero oxidation point. Krypton has many of the chemical properties and characteristics of some of the other noble gases.
The fragile compounds formed by noble gases at low temperatures, such as KrF 2 , are called clathrates.
AbundanceandSource Krypton is the 81st most abundant element on Earth and ranks seventh in abundance of
the gases that make up Earth’s atmosphere. It ranks just above methane (CH 4 ) in abundance in the atmosphere. Krypton is expensive to produce and thus has limited use. The gas is cap- tured commercially by fractional distillation of liquid air. Krypton shows up as an impurity in the residue. Along with some other gases, it is removed by filtering through activated charcoal and titanium.
270 | The History and Use of Our Earth’s Chemical Elements
There are traces of krypton in some minerals and meteorites. Krypton is found beyond Earth in space.
History In 1898 Sir William Ramsay (1852–1916), working with his graduate assistant, Morris W.
Travers (1872–1961), realized that there must be some other elements between the atomic weights of helium and argon (4 to 40) that would fit in group 18 (VIIIA), or what was then known as group 0. Thus, they attempted to identify the gases that would fill in this group. They improved their liquid-air instruments in an effort to capture the residues after liquid helium and argon were removed. It did not take them long to identify a new noble gas—kryp- ton—and a few weeks later, they discovered neon and xenon.
CommonUses Krypton is expensive to produce, which limits its use as an inert gas. It is used in a mixture
with argon to fill incandescent light bulbs, fluorescent lamps, lasers, and high-speed photog- raphy lamps. Radioactive Kr-85 is used as a source of radiation to measure the thickness of industrial materials. It is also used to test for “leakage” of scientific instruments.
Since 1960 the wavelength of the spectral lines of the krypton-86 isotope has been used as the standard for the length of the meter. One meter is now defined as 1,650,762.73 wave- lengths of the reddish-orange spectral line of the Kr-86 isotope.
ExamplesofCompounds There are no common compounds of krypton, but a few exotic compounds have been
formed at low temperatures. At temperatures of –220°C, krypton can be forced to com- bine with fluorine to form two different molecules: KrF 2 and KrF 4 . Both are unstable and decompose at temperatures higher than –220°C. They have been produced only in gram quantities.
Hazards Being an inert gas, krypton is nontoxic. However, the man-made radioisotopes of krypton
can cause radiation poisoning. XENON
SYMBOL:Xe PERIOD:5 GROUP:18(VIIIA) ATOMICNO:54 ATOMICMASS:131.293amu VALENCE:2,4,6,and8 OXIDATIONSTATE:0
NATURALSTATE:Gas ORIGINOFNAME:Theword“xenon”isderivedfromtheGreekwordxenon,meaning
“stranger.” ISOTOPES:Thereare46isotopesofxenon.Nineofthesearestable.Twoofthestable
isotopesareradioactive,butwithhalf-liveslongenoughtobeconsideredstable. TheyareXe-124(1.1×10 +17 years)andXe-136(3.6×10 +20 years).The47man- madeartificialradioactiveisotopeshavehalf-livesrangingfrom150nanosecondsto
Guide to the Elements |
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=18
s2,p6,d10
5-O=8
s2,p6
Properties Xenon has a relatively high atomic weight and is about 4.5 times heavier than air. It is col-
orless, tasteless, and odorless. Its critical temperature is comparatively high at 16.6°C, which is far above oxygen (–188°C). This means that xenon will boil away from commercial fractional distillation of liquid oxygen.
Xenon’s melting point is –111.79°C, its boiling point is –108.12°C, and its density is 0.005887g/cm 3 .
Characteristics Xenon is noncombustible, and even though it is considered inert, it will combine with a
few elements (i.e., oxygen, fluorine, and platinum). Xenon is the only member of group 18 that exhibits all of the even valence states of +2, +4, +6, and +8. It has similar oxidation states even though most periodic tables list a single oxidation state of zero.
AbundanceandSource Xenon is found in trace amounts in the atmosphere. It makes up just 0.086 ppm by volume
of air. Xenon is the rarest of the noble gases. For every thousand-million atoms of air, there are only 87 atoms of xenon. Even so, it is recovered in commercial amounts by boiling off the xenon from fractional distillation of liquid air. Small amounts of xenon have been found in some minerals and meteorites, but not in amounts great enough to exploit.
History Sir William Ramsay (1852–1916) and Morris William Travers (1872–1961) discovered
three new elements in just three months in 1898. They were krypton (May), neon (June), and xenon (July). The most difficult to identify was xenon because Ramsay and Travers needed to produce 10,000 pounds of liquid krypton in their refrigeration equipment in order to obtain just one pound of xenon. This was possible because of xenon’s high critical temperature and because xenon’s density is greater than oxygen’s.
CommonUses When excited electrically, xenon (sometimes mixed with krypton) produces a brilliant
272 | The History and Use of Our Earth’s Chemical Elements raphy can repeatedly be used to provide a well-balanced light for illumination. The xenon in
flash tubes is not consumed and can be flashed over and over again. Xenon lamps are also used as an antiseptic to kill bacteria, to power lasers, and as tracers. Because of its high atomic mass, xenon ions are preferred as fuel for ion engines to power spacecraft in deep space.
ExamplesofCompounds In the 1960s, scientists first produced compounds of xenon and some other noble gases at the
Argonne National Laboratory located near Chicago. Xenon and krypton are the only noble gases that readily form compounds with oxygen and fluorine. For instance, when xenon combines
with fluorine, it can form a series of compounds, such as xenon difluoride (XeF 2 ), xenon tetra- fluoride (XeF 4 ), and xenon hexafluoride (XeF 6 ). These and other compounds of xenon are formed within metal containers at high temperatures and pressures. They are not stable. Xenon tetraoxide (XeO 4 ) exhibits xenon with a +8 oxidation state. It is a very unstable and explosive gas. The ion of xenon has also been compounded with platinum to form XePtF 6 .
Hazards As a noble gas that is mostly inert, xenon is nontoxic and noncombustible. Some of its
compounds are toxic and potentially explosive, but there is little chance of coming into con- tact with them on a day-to-day basis.
RADON SYMBOL:Rn PERIOD:6 GROUP:18(VIIIA) ATOMICNO:86
ATOMICMASS:222amu VALENCE:+2,+4,and+6 OXIDATIONSTATE:0
NATURALSTATE:Gas ORIGINOFNAME:Originallynamed“niton”aftertheLatinwordfor“shining,”itwas
giventhename“radon”in1923becauseitistheradioactivedecaygasoftheelement radium.
ISOTOPES:Thereare37isotopesofradon.Allareradioactive.Nonearestable.Theyrange inmassnumbersfromRn-196toRn-228.Theirhalf-livesrangefromafewmicrosec- ondsto3.8235daysforRn-222,whichisthemostcommon.Itisagasthatistheresult ofalphadecayofradium,thorium,oruraniumoresandundergroundrocks.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=18
s2,p6,d10
6-P=8
s2,p6
273 Properties
Guide to the Elements |
Radon gas fits the criteria to be classed as a noble element located in group 18(VIIIA) or group 0. It is the only noble “inert” gas that is naturally radioactive. It is the heaviest of the gases in group 18.
Radon gas is easily converted to a liquid and will become solid at the relatively high temperature of –71°C. As a solid, it glows with a yellow light. Its melting point is –71°C, its
boiling point is –62°C, and its density is 0.00973g/cm 3 .
Characteristics Radon is the heaviest of the noble gases and is the only one that is radioactive. It is the
decay product of radium, thorium, and uranium ores and rocks found underground. As it decays, it emits alpha particles (helium nuclei) and is then transmuted to polonium and finally lead. The Earth’s atmosphere is just 0.0000000000000000001% radon, but because radon is 7.5 times heavier than air, it can collect in basements and low places in buildings and homes.
AbundanceandSource Radon’s source is a step in the transmutation of several elements: uranium → thorium →
radium → radon → polonium → lead. (There are a number of intermediate decay products and steps involved in this process.) Radon-222 forms and collects just a few inches below the surface of the ground and is often found in trapped pockets of air. It escapes through porous soils and crevices.
History While studying radium, Friedrich Ernst Dorn (1848–1916) found that it gave off a radio-
active gas that, when studied in more detail, proved to be the sixth noble gas. Dorn was given credit for its discovery in 1900. He called it “radon,” a variation of the word “radium.” Sir William Ramsay and R. W. Whytlaw-Gray, who also investigated the properties of radon, called it “niton” from the Latin word nitens, which means “shining.” Several other scientists who worked with radon named it “thoron” because of the transmutation of radon-220 from the decay of thorium. However, since 1923, the gas has been known as radon because it is the radioactive decay gas of the element radium. The name is derived from the Latin word radius, which means “ray.”
CommonUses Radon’s main use is as a short-lived source of radioactivity for medical purposes. It is
collected from the decay of radium as a gas and sealed in small glass capsules that are then inserted at the site of the cancer. It is also used to trace leaks in gas and liquid pipelines and to measure their rate of flow. The rate at which radon gas escapes from the Earth is one measure- ment that helps scientists predict earthquakes.
ExamplesofCompounds Because radon is inert and radioactive, there are not many useful compounds. The only one
confirmed so far is radon fluoride (RnF).