144 | The History and Use of Our Earth’s Chemical Elements Properties
144 | The History and Use of Our Earth’s Chemical Elements Properties
Cadmium is a soft, blue-white metal that is malleable and ductile although it becomes brittle at about 80°C. It is also found as a grayish-white powder. It is considered rare and is seldom found by itself as an ore. Its melting point at 320.9°C is considered low. Its boiling
point is 765°C, and its density is 8.65 g/cm 3 . Certain alloys of cadmium have extremely low melting points at about 70°C.
Characteristics Although cadmium is not considered a transition element in some periodic tables, it is the
central element of the triad with zinc and mercury. Zinc is just above it and mercury is below it in group 12 of the periodic table. Cadmium’s chemical and physical properties are similar to its group 12 mates. Their electronegativity is very similar: Zn = 1.6, Cd = 1.7, and Hg = 1.9.
Cadmium is resistant to alkalis, but is soluble in acids, mainly nitric acid. Although it is used to electroplate steel to prevent corrosion, it will tarnish in moist air.
AbundanceandSource Cadmium is considered a rare element even though it is widely distributed over the Earth’s
crust. Its estimated abundance in the Earth’s crust is 1.10 -1 milligrams per kilogram. It is con- sidered the 65th most abundant element, but it does not occur as a free metal in nature. It is usually found in relationship with other metallic ores. Its abundance is only about 1/1000th that of zinc. It is found in an ore called greenockite, which is cadmium sulfite (CdS). This ore does not have a high enough concentration of cadmium to be mined profitably. Cadmium is found along with zinc, lead, and copper ores. Today, most cadmium is obtained as a by- product from the processing and refining of zinc ores. In addition, dust and fumes from roast- ing zinc ores are collected by an electrostatic precipitator and mixed with carbon (coke) and sodium or zinc chloride. This residue is then treated to recover the cadmium. Other refining processes can obtain up to 40% recovery of cadmium from zinc ores.
Greenockite ore, as well as zinc and other ores, which produce cadmium as a by-product, are found in many countries, including Australia, Mexico, Peru, Zaire, Canada, Korea, and Belgium-Luxembourg and in the central and western United States.
History Several people in the 18th and 19th centuries attempted to produce a pure form of zinc
oxide for medical purposes. They were unaware that their samples contained cadmium, which at that time was an unknown element. In 1817 Friedrich Strohmeyer (1776–1835), a German chemist, analyzed a zinc compound (calamine) he believed contained zinc oxide (ZnO).
However, what he really found was zinc carbonate (ZnCO 3 ), which, though at first unknown to him, contained some cadmium. Strohmeyer then treated his sample with acids until all the zinc was dissolved and thus removed. He then heated the residue with carbon black, result- ing in a small ingot of soft, bluish-white metal that proved to be a new element—cadmium. Strohmeyer is given credit for the discovery of cadmium.
CommonUses Cadmium alloyed with silver forms a type of solder with a low melting point. It is used to
join electrical junctions and other specialized metallic components. Precautions are required
145 since it is a toxic substance. (Note: This is not the same as common solder used to join metals,
Guide to the Elements |
which is relatively safe.) Other cadmium alloys are used to manufacture long-wearing bearings and as thin coatings for steel to prevent corrosion.
Cadmium is a neutron absorber, making it useful as control rods in nuclear reactors. The rods are raised to activate the reactor and then lowered into the reactor to absorb neutrons that halt the fission reaction.
Cadmium, along with nickel, forms a nickel-cadmium alloy used to manufacture “nicad batteries” that are shaped the same as regular small dry-cell batteries. However, a major dif- ference is that the nicads can be recharged numerous times whereas the common dry cells cannot. A minor difference between the two types of cells is that nicads produce 1.4 volts, and regular carbon-zinc-manganese dioxide dry-cell batteries produce 1.5 volts.
Wood’s metal is another alloy that contains about 12.5% cadmium plus bismuth. It has a very low melting point of about 70°C, which makes it ideal for the “fuse” in overhead sprin- klers in hotels and office buildings. Any fire will melt the trigger-like fuse, opening the valve to jettison water spray over the hot area that melted the Wood’s metal alloy.
Several cadmium compounds—for example, CdSe, CdCl 2 and CdS—are used to make brilliant red-, yellow-, and orange-colored artists’ oil paints.
ExamplesofCompounds Cadmium has a single oxidation state of +2. Several examples follow:
Cadmium(II) bromide: Cd 2+ + 2Br 1- → CdBr 2 . This compound is used in photography, engraving, and lithography. The other halogen elements also combine with cadmium in a similar ionic reaction as with bromine.
Cadmium chloride (CdCl 2 ), a soluble crystal, is formed when cadmium metal is treated with hydrochloric acid (Cd + 2HCl → CdCl 2 +H 2 ). CdCl 2 is used in dyeing and printing textiles, in electroplating baths, in photography, and as the ingredient for cadmium yellow in artists’ oil paint. Cadmium(II) oxide: Cd 2+ +O 2- → CdO. This is used for cadmium plating baths, electrodes for batteries (cells), ceramic glazes, and insecticides. CdO is a deadly poison and carcinogen. Cadmium sulfate (CdS), also called “orange cadmium,” is used to produce phosphors and fluorescent screens. It is also used as a pigment in inks and paints, to color ceramics glazes, in the manufacture of transistors in electronics, photovoltaic cells, and solar cells, and in fireworks.
Cadmium tungstate (CdWO 4 ) is used in fluorescent paint, X-ray screens, and scintillation counters and as a catalyst. It is very toxic when inhaled. Many colored pigments are based on cadmium compounds. For instance, cadmium sulfide (CdS) and cadmium selenide (CdSe) are used as pigments when a durable, nonfading color is required. Red is produced by CdSe, and bright yellow is produced by CdS.
Cadmium compounds are also used for ceramics, TV and computer screens, transistors, photovoltaic cells, and solar cells.
Hazards Cadmium powder, dust, and fumes are all flammable and toxic if inhaled or ingested.
Cadmium and many of its compounds are carcinogenic. Severe illness and death can occur from exposure to many cadmium compounds. It is absorbed in the gastrointestinal tract. However, it can be eliminated in the urine and feces in young, healthy people.
146 | The History and Use of Our Earth’s Chemical Elements
Cadmium, in trace amounts, is common in our foods, and as we age, our bodies cannot eliminate it effectively, so cadmium poisoning may result. The symptoms of mild poisoning are burning of the eyes, irritation of the mouth and throat, and headaches. As the intoxica- tion increases, there may be severe coughing, nausea, vomiting, and diarrhea. There is a 15% chance of death from cadmium poisoning. The main risk from cadmium poisoning comes from industrial exposure—not from a healthy diet.
Transition Elements: Third Series—Period 6, Groups 4 to 12
HAFNIUM SYMBOL:Hf PERIOD:6 GROUP:4(IVB) ATOMICNO:72
ATOMICMASS:178.49amu VALENCE:2,3,and4 OXIDATIONSTATE:+4 NATURAL STATE:Solid ORIGINOFNAME:NamedafterHafnia,theLatinnameforthecityofCopenhagen,Den- mark. ISOTOPES:Thereare44knownisotopesforhafnium.Fivearestableandoneoftheunsta-
bleisotopeshassuchalonghalf-life(Hf-174witha2.0×10 +15 years)thatitisincluded ascontributing0.16%totheamountofhafniumfoundintheEarth’scrust.Thepercent- agecontributionsofthe5stableisotopestotheelement’snaturalexistenceonEarthare asfollows:Hf-176=5.26%,Hf-177=18.60%,Hf-178=27.28%,Hf-179=13.62%, andHf-180=35.08%.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=10
s2,p6,d2
6-P=2
s2
149 Properties
Guide to the Elements |
Hafnium is a ductile metal that looks and feels much like stainless steel, but it is signifi- cantly heavier than steel. When freshly cut, metallic hafnium has a bright silvery shine. When the fresh surface is exposed to air, it rapidly forms a protective oxidized coating on its sur- face. Therefore, once oxidized, hafnium resists corrosion, as do most transition metals, when exposed to the air. Chemically and physically, hafnium is very similar to zirconium, which is located just above it in group 4 on the periodic table. In fact, they are so similar that it is almost impossible to secure a pure sample of either one without a small percentage of the other. Each will contain a small amount of the other metal after final refining.
Hafnium’s melting point is 2,227°C, its boiling point varies from about 2,500°C to 5,000°C depending on its purity, and its density is 13.29 g/cm 3 . The compound hafnium nitride (HfN) has the highest melting point (over 3,300°C) of any two-element compound.
Characteristics As the first element in the third series of the transition elements, hafnium’s atomic number
( 72 Hf) follows the lanthanide series of rare-earths. The lanthanide series is separated out of the normal position of sequenced atomic numbers and is placed below the third series on the periodic table ( 57 La to 71 Li). This rearrangement of the table allowed the positioning of ele- ments of the third series within groups more related to similar chemical and physical charac- teristics—for example, the triads of Ti, Zr, and Hf; V, Nb, and Ta; and Cu, Ag, and Au.
AbundanceandSource Hafnium is the 47th most abundant element on Earth. Thus, it is more abundant than
either gold or silver. Because hafnium and zirconium are always found together in nature, both metals are refined and produced by the Kroll process. Pure samples of either hafnium or zir- conium are almost impossible to separate by the Kroll or other refining processes. Baddeleyite
(ZrO 2 ), a zirconium ore, and zircon (ZrSiO 4 ) are treated with chlorine along with a carbon catalyst that produces a mixture of zirconium and hafnium tetrachlorides. These are reduced by using sodium or magnesium, resulting in the production of both metals. The molten metals are separated by the process known as fractionation, which depends on their different melting points and densities. As the mixture of the two metals cools during the fractionation process, the denser solidified hafnium sinks to the bottom of the vessel while the less dense zirconium (with a higher melting point than hafnium) floats on top.
History Even though hafnium is not a scarce or rare element, it was not discovered until 1923
because of its close association with zirconium. Several scientists suspected that another ele- ment was mixed with zirconium but could not determine how to separate the two because zirconium ore contains about 50 times more zirconium than hafnium. Mendeleev predicted that there was an element with the atomic number of 72, but he predicted it would be found in titanium ore, not zirconium ore.
In 1923 Georg Karl von Hevesy (1885–1966) and Dirk Coster (1889–1950), on the advice of Danish physicist Niels Henrik Bohr (1885–1962), used X-ray spectroscopy to study the pattern of electrons in the outer shell of zirconium. Their analysis led to the discovery
150 | The History and Use of Our Earth’s Chemical Elements and identity of element 72, and thus, they are given credit for hafnium’s discovery. They also
named it after Hafnia, Latin name for the city of Copenhagen in Denmark, to honor Niels Bohr for his work on the quantum structure of matter and the science of spectroscopy. This discovery required some revision in the periodic table. Data on zirconium had to be reanalyzed and corrected, and the blank space of the new element with 72 protons in its nucleus could now be filled in.
CommonUses Hafnium has a great affinity for absorbing slow neutrons. This attribute, along with its
strength and resistance to corrosion, makes it superior to cadmium, which is also used for making control rods for nuclear reactors. This use is of particular importance for the type of nuclear reactors used aboard submarines. By moving the control rods in and out of a nuclear reactor, the fission chain reaction can be controlled as the neutrons are absorbed in the metal of the rods. The drawback to hafnium control rods is their expense: it costs approximately one million dollars for several dozen rods for use in a single nuclear reactor.
In vacuum tubes and other applications that must have gases removed, hafnium is used as
a “getter” to absorb any trace oxygen or nitrogen in the tube, thus extending the life of the vacuum tube. Hafnium’s qualities also make it ideal for filaments in light bulbs and, when mixed with rare-earth metals, as a “sparking” misch metal. Hafnium is also used to a lesser extent as an alloying agent for several other metals, including iron, titanium, and niobium.
ExamplesofCompounds Hafnium carbide (HfC): This alloy has one of the highest melting points of any binary
compound (3.890°C). It is extremely hard and resists corrosion while absorbing slow neu- trons. Therefore, it is an ideal metal in the manufacture of control rods for nuclear reactors.
Hafnium oxide (HfO 2 ): Resists heat and corrosion, making it an ideal lining for refractory furnaces. Following are two compounds formed by the main oxidation state of hafnium (+4), both of which are used in the refining and production of hafnium metal: Hafnium chloride: Hf 4+ + 4Cl → HfCl 4, also known as hafnium tetrachloride. Hafnium fluoride: Hf 4+ + 4F → HfF 4 , also known as hafnium tetrafluoride.
Hazards Although the metal hafnium is not harmful, its powder and dust are both toxic if inhaled
and explosive even when wet. TANTALUM
SYMBOL:Ta PERIOD:6 GROUP:5(VB) ATOMICNO:73 ATOMICMASS:180.948amu VALENCE:2,3,and5 OXIDATIONSTATE:+5 NATU-
RALSTATE:Solid ORIGINOFNAME:TantalumwasnamedafterTantalus,whowasthefatherofNiobe,the
queenofThebes,acityinGreekmythology.(Note:Theelementtantalumwasoriginally confusedwiththeelementnobelium.)
ISOTOPES:Thereare49isotopesoftantalum.OnlytheisotopeTa-181isstableand accountsfor99.988%ofthetotalmassoftheelementonEarth.Just0.012%ofthe
Guide to the Elements |
element’smassiscontributedbyTa-180,whichhasahalf-lifeof1.2×10 +15 yearsandis thusconsiderednaturallystable.Theremaining47isotopesareallartificiallyproduced innuclearreactionsorparticleacceleratorsandhavehalf-livesrangingfromafewmicro- secondstofewdaystoabouttwoyears.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=11
s2,p6,d3
6-P=2
s2
Properties Tantalum has properties similar to niobium and vanadium above it in group 5. It is a very
hard and heavy metal with a bluish color when in its rough state, but if polished, it has a sil- very shine. It is ductile, meaning it can be drawn into fine wires, and also malleable, meaning it can be hammered and worked into shapes. Thin strips and wires of tantalum will ignite in air if exposed to a flame.
Tantalum’s melting point is 2,996°C, which is almost as high as tungsten and rhenium. It
boiling point is 5,425°C, and its density is 19.3 g/cm 3 .
Characteristics Tantalum is almost as chemically inert at room temperatures (it has the ability to resist
chemical attacks, including hydrofluoric acid) as are platinum and gold. It is often substituted for the more expensive metal platinum, and its inertness makes it suitable for constructing dental and surgical instruments and artificial joints in the human body.
AbundanceandSource Tantalum is the 51st most abundant element found on Earth. Although it is found in a
free state, it is usually mixed with other minerals and is obtained by heating tantalum potas- sium fluoride or by the electrolysis of melted salts of tantalum. Tantalum is mainly obtained
from the following ores and minerals: columbite [(Fe, Mn, Mg)(Nb, Ta) 2 O 6 ]; tantalite [(Fe, Mn)(Ta, Nb) 2 O 6 ]; and euxenite [(Y, Ca, Er, La, Ce, U, Th)(Nb, Ta, Ti) 2 O 6 ]. Tantalum’s ores are mined in South America, Thailand, Malaysia, Africa, Spain, and Canada. The United States has a few small native deposits but imports most of the tantalum it uses.
Since tantalum and niobium are so similar chemically, a solvent process must be employed to separate them from the common ores. They are dissolved in a solvent, resulting in 98%
152 | The History and Use of Our Earth’s Chemical Elements pure niobium oxide being extracted during this part of the process. This is followed by 99.5%
pure tantalum oxide being extracted in a second solvent process History
Anders Gustav Ekeberg (1767–1813) discovered tantalum in 1802, while analyzing ores sent to him by his friend Jons Jakob Berzelius (1779–1848) from the famous mineral deposits of Ytterby, Sweden. At first, it was thought that this new element was an allotrope (close rela- tive) of niobium because they were so similar in physical and chemical characteristics. Ekeberg named it tantalum after the Greek King Tantalus, who was condemned to everlasting torment. The word means “to tantalize.” Tantalum was not separated, analyzed, and identified as a sepa- rate element with an atomic number 72 until 1866, by Jean Charles Galissard de Marignac (1817–1894) of Switzerland. He proved that tantalum and niobium are two different and distinct elements. The first pure samples were not produced until the year 1907.
CommonUses
A mixture of tantalum carbide (TaC) and graphite is a very hard material and is used to form the cutting edge of machine tools. Tantalum pentoxide (Ta 2 O 5 ) is dielectric, making it useful to make capacitors in the electronics industry. When mixed with high-quality glass, it imparts a high index of refraction, making it ideal for camera and other types of lenses.
Because of its hardness and noncorrosiveness, tantalum is used to make dental and sur- gical tools and implants and artificial joints, pins, and screws. The metal does not interact with human tissues and fluids. Since tantalum can be drawn into thin wires, it is used in the electronics industry, to make smoke detectors, as a getter in vacuum tubes to absorb residual gases, and as filaments in incandescent lamps. It has many other uses in the elec- tronics industry.
The use of tantalum to make miniaturized electrolytic capacitors that store electric charges in devices such as cell phones and computers is becoming increasingly popular. Powdered tantalum is used in the process of sintering to form malleable bars and plates as well as special electrodes for the electronics industry.
As a result of their hardness, noncorrosiveness, and ductility, tantalum alloys are used to fabricate parts for nuclear reactors, missiles, and airplanes, and in industries where metal with these qualities is required.
ExamplesofCompounds Tantalum pentoxide is representative of tantalum’s stable oxidation state of +5: 2Ta 5+ + 5O 2-
→ Ta 2 O 5 . Tantalum oxide is used to make optical glass for lenses and in electronic circuits. Tantalum carbide (TaC) is one of the hardest substances known. This compound represents its oxidation state of +4 for tantalum. Tantalum disulfide (TaS 2 ) is used to make solid lubricants and special noncorrosive greases. Tantalum fluoride (TaF 5 ) is a catalyst used to speed up organic chemical reactions.
Tantalum pentoxide (Ta 2 O 5 ) is used to make special optical glass, for lasers, and in electronic circuits.
Hazards The dust and powder of tantalum are explosive. Several tantalum compounds are toxic if
153 TUNGSTEN
Guide to the Elements |
SYMBOL:W PERIOD:6 GROUP:6(VIB) ATOMICNO:74 ATOMICMASS:183.85amu VALENCE:2,4,5,and6 OXIDATIONSTATE:+4,and+6
NATURALSTATE:Solid ORIGINOFNAME: Tungstenwasoriginallynamed“Wolfram”byGermanscientists,after themineralinwhichitwasfound,Wolframite—thus,itssymbol“W.” Later,Swedishscien- tistsnamedittungsten,whichmeans“heavystone,”butitretaineditsoriginalsymbol of“W.”
ISOTOPES:Thereare36isotopesoftungsten.Fivearenaturallystableandthereforecon- tributeproportionallytotungsten’sexistenceonEarth,asfollows:W-180=0.12%,W-
182=26.50%,W-183=14.31%,W-184=30.64%,andW-186=28.43%.Theother 31isotopesareman-madeinnuclearreactorsandparticleacceleratorsandhavehalf- livesrangingfromfractionsofasecondtomanydays.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=12
s2,p6,d4