138 | The History and Use of Our Earth’s Chemical Elements
138 | The History and Use of Our Earth’s Chemical Elements
ISOTOPES:Thereare42isotopesofpalladium,rangingfromPd-91toPd-124.Allbutsix areradioactiveandartificiallyproducedinnuclearreactorswithhalf-livesrangingfrom 159nanosecondsto6.5×10 +6 years.Thesixstableisotopesofpalladiumandtheir proportionalcontributiontotheirexistenceintheEarth’scrustareasfollows:Pd-102= 1.02%,Pd-104=11.14%,Pd-105=22.23%,Pd-106=27.33%,Pd-108=26.46%, andPd-110=11.72%.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=18
s2,p6,d10
5-O=0
s0
Properties Palladium is the middle element in group 10 of the transition elements (periods 4, 5, and
6). Many of its properties are similar to nickel located above it and platinum just below it in this group.
Palladium is a soft, silvery-white metal whose chemical and physical properties closely resemble platinum. It is mostly found with deposits of other metals. It is malleable and ductile, which means it can be worked into thin sheets and drawn through a die to form very thin wires. It does not corrode. Its melting point is 1,554°C, its boiling point is 3,140°C, and its
density is 12.02 g/cm 3 .
Characteristics One of palladium’s unique characteristics is its ability to absorb 900 times its own volume
of hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H 2 ), the gas molecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in the crystal structure of the metal. The result is a metallic hydride (PdH 0.5 ) that changes palladium from an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl 2 ) also has the ability to absorb large quantities of carbon monoxide (CO). These characteristics are useful for many commercial applications. Palladium is the most reactive of all the platinum family of elements (Ru, Rh, Pd, Os, Is, and Pt.)
AbundanceandSource Palladium is considered a rare metal, making up only about 1 part per 100 million parts
of the Earth’s crust. It is considered the 77th most abundant element on Earth, although it is
139 seldom found in pure states. Rather, it is mixed with other metals or in compounds of pal-
Guide to the Elements |
ladium. It was originally found in gold ores from Brazil, where the miners thought the gold was contaminated by what they referred to as “white gold.” Later, it was considered an alloy com- bination of palladium and gold.
Deposits of ores containing palladium, as well as other metals, are found in Siberia and the Ural Mountains of Russia, Canada, and South Africa, as well as in South America.
History Several scientists in the early 1800s were aware that there were other elements mixed with
platinum and other ores (i.e., nickel, copper, silver, and gold). They had difficulty separating the metals because these elements (metals) were so similar in their physical and chemical characteristics.
In 1803 William Hyde Wollaston (1776–1828), an English chemist who also discovered rhodium, isolated palladium at the time he analyzed the platinum and gold ores sent to him from Brazil. Dissolving the platinum in aqua regia acid, Wollaston then treated the residue with mercuric cyanide to produce the compound of palladious cyanide that was reduced by burning it to extract metallic palladium.
CommonUses Palladium’s ability to absorb large amounts of hydrogen makes it an excellent catalyst for
chemical reactions as well as catalytic converters for internal combustion engines. Palladium is also an excellent catalyst for cracking petroleum fractions and for hydrogenation of liq- uid vegetable oils into solid forms, such as corn oil into margarine. It is also used to purify
hydrogen gas by passing raw H 2 gas under pressure through thin sheets of palladium, where the pure hydrogen passes through the metal’s crystal structure, leaving behind impurities. Palladium is used to manufacture CO-monitoring devices because of its ability to absorb carbon monoxide.
Palladium is used in the manufacture of surgical instruments, electrical contacts, springs for watches and clocks, high-quality spark plugs, and special wires and as “white gold” in jewelry. Because it is noncorrosive, it is used as a coating for other metals and to make dental fillings and crowns.
ExamplesofCompounds The most common oxidation states of palladium are +2 and +4. Some examples follow:
Palladium(II) oxide: Pd 2+ +O 2- → PdO. Palladium(IV) dioxide: Pd 4+ + 2O 2- → PdO 2. When combining with the halogens, the oxidation states of +2 or +4 for palladium are
used.
Palladium(IV) fluoride: Pd 4+ +F 1- → PdF 4 , a brick-red color.
Palladium(II) chloride: Pd 2+ + Cl 1- → PdCl 2 . This compound is a dark brown color and is used to coat other metals without the need for electrolysis. It is also used in photography, to make indelible inks, and as a catalyst in analytical chemistry (used to speed up or slow down chemical reactions).
140 | The History and Use of Our Earth’s Chemical Elements
Palladium(II) bromide: Pd 1- + Br → PdBr
2 , brownish-black color.
2 , black in color. Palladium sodium chloride: NaPdCl 2 . This compound is used to test for the presence of such gases as carbon monoxide, illuminating and cooking gas, and ethylene and for the presence of iodine.
Palladium(II) iodide: Pd 1- +I → PdI
Hazards Palladium is not combustible except as fine powder or dust. Several of palladium’s com-
pounds are oxidizing agents, and some react violently with organic substances. SILVER SYMBOL:Ag PERIOD:5 GROUP:11(1B) ATOMICNO:47
ATOMICMASS:107.868amu VALENCE:1 OXIDATIONSTATE:+1 NATURALSTATE: Solid ORIGINOFNAME:Silver’smodernchemicalsymbol(Ag)isderivedfromitsLatinword argentum,whichmeanssilver.Theword“silver”isfromtheAnglo-Saxonworld“siolfor.” Ancientswhofirstrefinedandworkedwithsilverusedthesymbolofacrescentmoonto representthemetal.
ISOTOPES:Thereare59isotopesofsilver,rangingfromAg-93toAg-130withhalf-lives fromafewmillisecondstoafewdaysto418years.Allbuttwoofthese59isotopesare radioactiveandareproducedsynthetically.Thetwostableisotopesfoundinnatureare Ag-107andAg-109.Thesetwomakeup100%oftheelement’sexistenceintheEarth’s crust.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=18
s2,p6,d10
5-O=1
s1
Properties Silver is located in group 11 (IB) of period 5, between copper (Cu) above it in period 4 and
gold (Au) below it in period 6. Thus, silver’s chemical and physical properties are somewhat similar to these two group 11 partners.
Silver is a soft, while, lustrous metal that can be worked by pounding, drawing through
a die, rolling, and so forth. It is only slightly harder than gold. It is insoluble in water, but
141 it will dissolve in hot concentrated acids. Freshly exposed silver has a mirror-like shine that
Guide to the Elements |
slowly darkens as a thin coat of tarnish forms on its surface (from the small amount of natural hydrogen sulfide in the air to form silver sulfide, AgS). Of all the metals, silver is the best conductor of heat and electricity. This property determines much of its commercial usefulness. Its melting point is 961.93°C, its boiling point is 2,212°C, and its density is
10.50 g/cm 3 . Characteristics
Silver is somewhat rare and is considered a commercially precious metal with many uses. Pure silver is too soft and usually too expensive for many commercial uses, and thus it is alloyed with other metals, usually copper, making it not only stronger but also less expensive. The purity of silver is expressed in the term “fitness,” which describes the amount of silver in the item. Fitness is just a multiple of 10 times the silver content in an item. For instance, sterling silver should be 93% (or at least 92.5%) pure silver and 7% copper or some other metal. The fitness rating for pure silver is 1000. Therefore, the rating for sterling silver is 930, and most sliver jewelry is rated at about 800. This is another way of saying that most silver jewelry is about 20% copper or other less valuable metal.
Many people are fooled when they buy Mexican or German silver jewelry, thinking they are purchasing a semiprecious metal. These forms of “silver” jewelry go under many names, including Mexican silver, German silver, Afghan silver, Austrian silver, Brazilian silver, Nevada silver, Sonara silver, Tyrol silver, Venetian silver, or just the name “silver” with quotes around it. None of these jewelry items, under these names or under any other names, contain any silver. These metals are alloys of copper, nickel, and zinc.
AbundanceandSource Silver is the 66th most abundant element on the Earth, which means it is found at about
0.05 ppm in the Earth’s crust. Mining silver requires the movement of many tons of ore to recover small amounts of the metal. Nevertheless, silver is 10 times more abundant than gold. And though silver is sometimes found as a free metal in nature, mostly it is mixed with the ores of other metals. When found pure, it is referred to as “native silver.” Silver’s major ores are
argentite (silver sulfide, Ag 2 S) and horn silver (silver chloride, AgCl). However, most silver is recovered as a by-product of the refining of copper, lead, gold, and zinc ores. Although silver is mined in many countries, including the United States, Mexico, and Canada, most silver is recovered from the electrolytic processing of copper ores. Silver can also be recovered through the chemical treatment of a variety of ores.
History Silver is probably one of the first metallic elements used by humans. It was known to primi-
tive humans before 5000 BCE, at about the time copper and gold were also found in native “free” forms. Silver jewelry was found in tombs that are over 6,000 years old.
It was not until about 4000 BCE that humans learned how to obtain these base metals from ores by using heat. Both the refining of and the uses for silver as jewelry are described in ancient Egyptian writings as well as the Old Testament of the Bible. During these ancient times, silver was more valuable than gold primarily because it was more difficult to find in its natural state as well as to extract from its ores.