184 | The History and Use of Our Earth’s Chemical Elements Hazards
184 | The History and Use of Our Earth’s Chemical Elements Hazards
Most gallium compounds are toxic, particularly the metal gallium arsenide. When forms of gallium are used in the electronics industry, great care must be taken to protect workers.
INDIUM SYMBOL:In PERIOD:5 GROUP:13(IIIA) ATOMICNO:49
ATOMICMASS:114.818amu VALENCE:1and3 OXIDATIONSTATE:+3
NATURALSTATE:Solid ORIGINOFNAME: Indium’snameisderivedfromtheLatinwordindicum,meaning“indi-
go,”whichisthecolorofitsspectrallinewhenviewedbyaspectroscope. ISOTOPES:Thereareatotalof73isotopesofindium.Allareradioactivewithrelatively
shorthalf-lives,excepttwothatareconsideredstable.IsotopeIn-113makesupjust 4.29%ofthetotalindiumfoundintheEarth’scrust.TheisotopeIn-115,withahalf-life of4.41×10 -14 yearscontributesthebalance(95.71%)oftheelement’sexistenceinthe
Earth’scrust.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=18
s2,p6,d10
5-O=3
s2,p1
Properties Indium is silvery-white and malleable and looks much like aluminum and tin. However,
it is softer than lead. Indium metal is so soft that it cannot be “wiped” onto other surfaces as with a graphite pencil. Because it is noncorrosive and does not oxidize at room temperatures, it can be polished and will hold its shine better than silver. Its melting point is 156.60°C, its
boiling point is 2,075°C, and its density is 7.31 g/cm 3 .
Characteristics Indium has one odd characteristic in that in the form of a sheet, like the metal tin, it will
emit a shrieking sound when bent rapidly. Indium has some of the characteristics of other metals near it in the periodic table and may be thought of as an “extension” of the second series of the transition elements. Although it is corrosion-resistant at room temperature, it will
185 AbundanceandSource
Guide to the Elements |
Indium is a rather rare metal. It is the 69th most abundant element, which is about as abundant as silver at 0.05 ppm. Although it is widely spread over the Earth’s crust, it is found in very small concentrations and always combined with other metal ores. It is never found in its natural metallic state.
Indium is recovered as a by-product of smelting other metal ores such as aluminum, antimony, cadmium, arsenic, and zinc. About 1,000 kg of indium is recovered each year (or
a concentration of 1 part indium per 1000 parts of dust) from the flue stacks (chimneys) of zinc refineries. Indium is found in metal ores and minerals located in Russia, Japan, Europe, Peru, and Canada, as well as in the western part of the United States.
History While searching for traces of one element contained in some zinc ore, Ferdinand Reich
(1799–1882) in 1863 accidentally produced a yellow sulfide of what he suspected was a new element. By this time in history the spectroscope was being used to identify elements by their unique color spectrum. Reich was colorblind and could not use the instrument, but he asked his assistant, Hieronymous Theodor Richter (1824–1898), to examine the new element with the spectroscope. (A spectroscope is an instrument used to analyze the wavelength of light emitted or absorbed by elements when excited. Each element has a unique line or color in the light spectrum.) Richter saw the separate deep-purple line of the new element, and together Reich and Richter became the codiscoverers of indium, which they named after the indigo plant that grew in India, as well as in Egypt and China.
The indigo plant was used as early as 3000 BCE to make purple dyes. For many years humans used insects, snails, and plants to make dyes of bright red, deep red, purple, brown, yellow, and black. The processes of extracting the dyes were expensive. Only the wealthy, and primarily royalty at this early time in history, could afford the deep purple dyes for their cloth- ing and robes. Thus, the tradition of purple as the color of kings was originated.
By the late 1800s, many chemists were trying to synthetically produce dyes such as indigo, but it was not until 1900 that synthetic indigo production was a commercial success. The syn- thetic production of indigo and other synthetic dyes, most notably the color mauve, destroyed the commercial agriculture trade of the extensive indigo-growing regions of India and other countries that depended on growing the plants for their livelihood.
CommonUses Indium’s low melting point is the major factor in determining its commercial importance.
This factor makes it ideal for soldering the lead wires to semiconductors and transistors in the electronics industry. The compounds of indium arsenide, indium antimonide, and indium phosphide are used to construct semiconductors that have specialized functions in the elec- tronics industry.
Another main use is as an alloy with other metals when it will lower the melting point of the metals with which it is alloyed. Alloys of indium and silver and indium and lead have the ability to carry electricity better than pure silver and lead.
Indium is used as a coating for steel bearings to increase their resistance to wear. It also
186 | The History and Use of Our Earth’s Chemical Elements mercury mirrors. Sheets of indium foil are inserted into nuclear reactors to help control the
nuclear fission reaction by absorbing some of the neutrons. ExamplesofCompounds
Indium’s stable oxidation state is +3 and thus, being trivalent, it forms compounds with elements with –1, –2, and –3 oxidation states as follows: Indium trichloride (In 3+ + 3Cl 1- → InCl 3 ). Hazardous: keep container closed. Indium trioxide (2In 3+ + 3O 2- →In 2 O 3 ). Used in specialty glass production. Indium arsenide (In 3+ + As 3- → InAs). Used in semiconductor devices. There are many other compounds of indium, most of which are useful in the electronics
and semiconductor industries. Some other examples are InP, In 2 (SO 4 ) 3 , In 2 Te 3 , and InSb. Hazards
Indium metal dust, particles, and vapors are toxic if ingested or inhaled, as are most of the compounds of indium. This requires the semiconductor and electronics industries that use indium compounds to provide protection for their workers.
THALLIUM SYMBOL:Tl PERIOD:6 GROUP:13(IIIA) ATOMICNO:81
ATOMICMASS:204.383amu VALENCE:1and3 OXIDATIONSTATE:+1and
+3 NATURALSTATE:Solid ORIGINOFNAME:FromtheGreekwordthallos,meaning“youngshoot”or“greentwig.” Namedforthegreenspectrallineproducedbythelightfromtheelementinaspectro- scope.
ISOTOPES:Thereareatotalof55isotopesforthallium.Allareradioactivewithrelatively shorthalf-lives,andonlytwoarestable.ThestableonesareTl-203,whichconstitutes 29.524%oftheelement’sexistenceintheEarth’scrust,andTl-205,whichmakesup 70.476%oftheelement’snaturalabundancefoundintheEarth’scrust.
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=3
s2,p1
187 Properties
Guide to the Elements |
Thallium has much the same look (silvery) and feel as lead and is just as malleable. Unlike lead, which does not oxidize readily, thallium will oxidize in a short time, first appearing as a dull gray, then turning brown, and in just a few years or less turning into blackish corroded chunks of thallium hydroxide. This oxide coating does not protect the surface of thallium because it merely flakes off exposing the next layer to oxidation.
Thallium is just to the left of lead in period 6, and both might be considered extensions of the period 6 transition elements. Thallium’s high corrosion rate makes it unsuitable for most commercial applications. Its melting point is 304°C, its boiling point is 1,473°C, and
its density is 11.85 g/cm 3 . Characteristics Elemental thallium metal is rare in nature mainly because it oxidizes if exposed to air (oxy-
gen) and water vapor, forming thallium oxide, a black powder. Although some compounds of thallium are both toxic and carcinogenic, they have some uses in the field of medicine. Some compounds have the ability to alter their electrical conductivity when exposed to infrared light.
AbundanceandSource Thallium is the 59th most abundant element found in the Earth’s crust. It is widely dis-
tributed over the Earth, but in very low concentrations. It is found in the mineral/ores of crooksite (a copper ore; CuThSe), lorandite (TlAsS 2 ), and hutchinsonite (lead ore, PbTl). It is found mainly in the ores of copper, iron, sulfides, and selenium, but not in its elemental metallic state. Significant amounts of thallium are recovered from the flue dust of industrial smokestacks where zinc and lead ores are smelted.
History Prior to its discovery, no one knew that thallium existed, and thus, no one was looking
for it. Therefore, it was discovered by accident by Sir William Crookes (1832–1919) in 1861. After removing selenium from a sample of minerals, he observed the sample with his spectro- scope, expecting to see some blue lines of leftover selenium and the yellow lines of tellurium, but instead he observed a never-before-seen bright green line, which indicated a new element. This green line inspired him to name the new element after the Greek word thallos, which stands for “green twig.”
CommonUses Thallium is used as an alloy with mercury and other metals. One main use is in photoelec-
tric applications and for military infrared radiation transmitters. It is also used to make artificial gemstones and special glass and to make green colors in fireworks and flares. It formerly was used as a rat poison, but is no longer used for this purpose because it is very toxic to humans.
Another main use is the radioisotope TlCl-201, with the relatively short half-life of about
73 hours, in cardiac stress tests to identify potential heart abnormalities. TlCl-201 has an ability to bind with the heart muscle, but only if the heart is receiving an adequate supply of
188 | The History and Use of Our Earth’s Chemical Elements blood. Restricted blood flow by blocked or narrow arteries in the heart limits the supply of
TlCl-201 absorbed. First, a small dose of TlCl-201 is injected into the patient, and the patient then engages in a strenuous workout on a treadmill. Both before and after the test, the patient is scanned by a “gamma” detector that sends the results to a computer where the physician can compare the uptake of TlCl-201 before and after the treadmill stress test to determine the condition of the patient’s heart. An area where the heart’s muscle is weak and the blood flow is limited will show up as a darkish spot on the computer screen. Since the radioisotope TlCl- 201 has such a short half-life, it is soon excreted from the body. Thus, there are no long-term detriments to the body.
ExamplesofCompounds Two examples of compounds in thallium +1 and +3 oxidation states follow:
Thallium (I) hydroxide (Tl 1+ + OH 1- → TlOH) is used in optical glass.
Thallium (III) chloride: Tl 3+ +3Cl 1- → TlCl 3 .
A few other compounds include the following: Thallium carbonate (Tl 2 CO 3 ) is used to make artificial diamonds (along with several other thallium compounds). Thallium sulfide (Tl 2 S) is used to make infrared-sensitive photoelectric cells and as a pes- ticide. Thallium-mercury is an amalgam—not really a compound, but more like an alloy mixture. It is used in low-temperature thermometers and as a substitute for mercury in low-tempera- ture switches.
Hazards In all forms, thallium is very toxic if inhaled, when in contact with the skin, and in par-
ticular, if ingested. Mild thallium poisoning causes loss of muscle coordination and burning of the skin, followed by weakness, tremor, mental aberration, and confusion.
Thallium disease (thallotoxicosis) results from the ingestion of relatively large doses (more than a few micrograms). The severity may vary with the age and health of the patient. Nerves become inflamed, hair is lost, the patient experiences stomach pain, cramps, hemorrhage, rapid heartbeat, delirium, coma, and respiratory paralysis. The disease has the potential to cause death in about one week. In the past thallium was one of the poisons of choice used by murderers because it acts slowly and makes victims suffer. In 1987 then Iraqi dictator Saddam Hussein’s agents mixed thallium powder in orange juice or yogurt and fed it to people he perceived to be his enemies. There were at least 40 thallium poisonings, mostly of Kurdish leaders. (William Langewiesche, “The Accuser,” Atlantic, March 2005, 56.)
The Carbon Group (Metalloids to Semiconductors): Periods 2 to 6, Group 14 (IVA)