Transition Metals

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TRANSITION METALS

CONCEPT

By far the largest family of elements is the one known as the transition metals, sometimes called transition elements. These occupy the "dip" in the periodic table between the "tall" sets of columns or groups on either side. Consisting of 10 columns and four rows or periods, the transition metals are usually numbered at 40. With the inclusion of the two rows of transition metals in the lanthanide and actinide series respectively, however, they account for 68 elements—considerably more than half of the periodic table. The transition metals include some of the most widely known and commonly used elements, such as iron—which, along with fellow transition metals nickel and cobalt, is one of only three elements known to produce a magnetic field. Likewise, zinc, copper, and mercury are household words, while cadmium, tungsten, chromium, manganese, and titanium are at least familiar. Other transition metals, such as rhenium or hafnium, are virtually unknown to anyone who is not scientifically trained. The transition metals include the very newest elements, created in laboratories, and some of the oldest, known since the early days of civilization. Among these is gold, which, along with platinum and silver, is one of several precious metals in this varied family.

HOW IT WORKS

Two Numbering Systems for the Periodic Table

The periodic table of the elements, developed in 1869 by Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907), is discussed in several places within this volume. Within the table, elements are arranged in columns, or groups, as well as in periods, or rows.

As discussed in the Periodic Table of Elements essay, two basic versions of the chart, differing primarily in their method of number groups, are in use today. The North American system numbers only eight groups—corresponding to the representative or main-group elements—leaving the 10 columns representing the transition metals unnumbered. On the other hand, the IUPAC system, approved by the International Union of Pure and Applied Chemistry (IUPAC), numbers all 18 columns. Both versions of the periodic table show seven periods.

In general, this book applies the North American system, in part because it is the version used in most American classrooms. Additionally, the North American system relates group number to the number of valence electrons in the outer shell of the atom for the element represented. Hence the number of valence electrons (a subject discussed below as it relates to the transition metals) for Group 8 is also eight. Yet where the transition metals are concerned, the North American system is less convenient than the IUPAC system.

ATTEMPTS TO ADJUST THE NORTH AMERICAN SYSTEM.

In addressing the transition metals, there is no easy way to apply the North American system's correspondence between the number of valence electrons and the group number. Some versions of the North American system do, however, make an attempt. These equate the number of valence electrons in the transition metals to a group number, and distinguish these by adding a letter "B."

Thus the noble gases are placed in Group 8A or VIIIA because they are a main-group family with eight valence electrons, whereas the transition metal column beginning with iron (Group 8 in the IUPAC system) is designated 8B or VIIIB. But this adjustment to the North American system only makes things more cumbersome, because Group VIIIB also includes two other columns—ones with nine and 10 valence electrons respectively.

VALUE OF THE IUPAC SYSTEM.

Obviously, then, the North American system—while it is useful in other ways, and as noted, is applied elsewhere in this book—is not as practical for discussing the transition metals. Of course, one could simply dispense with the term "group" altogether, and simply refer to the columns on the periodic table as columns. To do so, however, is to treat the table simply as a chart, rather than as a marvelously ordered means of organizing the elements.

It makes much more sense to apply the IUPAC system and to treat the transition metals as belonging to groups 3 through 12. This is particularly useful in this context, because those group numbers do correspond to the numbers of valence electrons. Scandium, in group 3 (hence-forth all group numbers refer to the IUPAC version), has three valence electrons; likewise zinc, in Group 12, has 12. Neither the IUPAC nor the North American system provide group numbers for the lanthanides and actinides, known collectively as the inner transition metals.

THE TRANSITION METALS BY IUPAC GROUP NUMBER.

Here, then, is the list of the transition metals by group number, as designated in the IUPAC system. Under each group heading are listed the four elements in that group, along with the atomic number, chemical symbol, and atomic mass figures for each, in atomic mass units.

Note that, because the fourth member in each group is radioactive and sometimes exists for only a few minutes or even seconds, mass figures are given merely for the most stable isotope. In addition, the last elements in groups 8, 9, and 10, as of 2001, had not received official names. For reasons that will be explained below, lanthanum and actinium, though included here, are discussed in other essays.

Group 1

21. Scandium (Sc): 44.9559
39. Yttrium (Y): 88.9059
57. Lanthanum (La): 138.9055
89. Actinium (Ac): 227.0278

Group 2

22. Titanium (Ti): 47.9
40. Zirconium (Zr): 91.22
72. Hafnium (Hf): 178.49
104. Rutherfordium (Rf): 261

Group 3

23. Vanadium (V): 50.9415
41. Niobium (Nb): 92.9064
73. Tantalum (Ta): 180.9479
105. Dubnium (Db): 262

Group 4

24. Chromium (Cr): 51.996
42. Molybdenum (Mo): 95.95
74. Tungsten (W): 183.85
106. Seaborgium (Sg): 263

Group 5

25. Manganese (Mn): 54.938
43. Technetium (Tc): 98
75. Rhenium (Re): 186.207
107. Bohrium (Bh): 262

Group 6

26. Iron (Fe): 55.847
44. Ruthenium (Ru): 101.07
76. Osmium (Os): 190.2
108. Hassium (Hs): 265

Group 7

27. Cobalt (Co): 58.9332
45. Rhodium (Rh): 102.9055
77. Iridium (Ir): 192.22
109. Meitnerium (Mt): 266

Group 8

28. Nickel (Ni): 58.7
46. Palladium (Pd): 106.4
78. Platinum (Pt): 195.09
110. Ununnilium (Uun): 271

Group 9

29. Copper (Cu): 63.546
47. Silver (Ag): 107.868
79. Gold (Au): 196.9665
111. Unununium (Uuu): 272

Group 10

30. Zinc (Zn): 65.38
48. Cadmium (Cd): 112.41
80. Mercury (Hg): 200.59
112. Ununbium (Uub): 277

Electron Configurations and the Periodic Table

We can now begin to explain why transition metals are distinguished from other elements, and why the inner transition metals are further separated from the transition metals. This distinction relates not to period (row), but to group (column)—which, in turn, is defined by the configuration of valence electrons, or the electrons that are involved in chemical bonding. Valence electrons occupy the highest energy level, or shell, of the atom—which might be thought of as the orbit farthest from the nucleus. However, the term "orbit" is misleading when applied to the ways that an electron moves.

Electrons do not move around the nucleus of an atom in regular orbits, like planets around the Sun; rather, their paths can only be loosely defined in terms of orbitals, a pattern of probabilities indicating the regions that an electron may occupy. The shape or pattern of orbitals is determined by the principal energy level of the atom, which indicates a distance that an electron may move away from the nucleus.

Principal energy level defines period: elements in Period 4, for instance, all have their valence electrons at principal energy level 4. The higher the number, the further the electron is from the nucleus, and hence, the greater the energy in the atom. Each principal energy level is divided into sublevels corresponding to the number n of the principal energy level: thus, principal energy level 4 has four sublevels, principal energy level 5 has five, and so on.

ORBITAL PATTERNS.

The four basic types of orbital patterns are designated as s, p, d, and f. The s shape might be described as spherical, which means that in an s orbital, the total electron cloud will probably end up being more or less like a sphere.

The p shape is like a figure eight around the nucleus; the d like two figure eights meeting at the nucleus; and the f orbital pattern is so complex that most basic chemistry textbooks do not even attempt to explain it. In any case, these orbital patterns do not indicate the exact path of an electron. Think of it, instead, in this way: if you could take millions of photographs of the electron during a period of a few seconds, the resulting blur of images in a p orbital, for instance, would somewhat describe the shape of a figure eight.

Since the highest energy levels of the transition metals begin at principal energy level 4, we will dispense with a discussion of the first three energy levels. The reader is encouraged to consult the Electrons essay, as well as the essay on Families of Elements, for a more detailed discussion of the ways in which these energy levels are filled for elements on periods 1 through 3.

Orbital Filling and the Periodic Table

If all elements behaved as they "should," the pattern of orbital filling would be as follows. In a given principal energy level, first the two slots available to electrons on the s sublevel are filled, then the six slots on the p sublevel, then the 10 slots on the d sublevel. The f sublevel, which is fourth to be filled, comes into play only at principal energy level 4, and it would be filled before elements began adding valence electrons to the s sublevel on the next principal energy level.

That might be the way things "should" happen, but it is not the way that they do happen. The list that follows shows the pattern by which orbitals are filled from sublevel 3p onward. Following each sublevel, in parentheses, is the number of "slots" available for electrons in that shell. Note that in several places, the pattern of filling defies the ideal order described in the preceding paragraph.

Orbital Filling by Principal Energy Level and Sublevel from 3p Onward:

3p (6)
4s (2)
3d (10)
4p (6)
5s (2)
4d (10)
5p (6)
6s (2)
4f (14)
5d (10)
6p (6)
7s (2)
5f (14)
6d (10)

The 44 representative elements follow a regular pattern of orbital filling through the first 18 elements. By atomic number 18, argon, 3p has been filled, and at that point element 19 (potassium) would be expected to begin filling row 3d. However, instead it begins filling 4s, to which calcium (20) adds the second and last valence electron. It is here that we come to the transition metals, distinguished by the fact that they fill the d orbitals.

Note that none of the representative elements up to this point, or indeed any that follow, fill the d orbital. (The d orbital could not come into play before Period 3 anyway, because at least three sublevels—corresponding to principal energy level 3—are required in order for there to be a d orbital.) In any case, when the representative elements fill the p orbitals of a given energy level, they skip the d orbital and go on to the next principal energy level, filling the s orbitals. Filling of the d orbital on the preceding energy level only occurs with the transition metals: in other words, on period 4, transition metals fill the 3d sublevel, and so on as the period numbers increase.

Distinguishing the Transition Metals

Given the fact that it is actually the representative elements that skip the d sublevels, and the transition metals that go back and fill them, one might wonder if the names "representative" and "transition" (implying an interruption) should be reversed. But it is the transition metals that are the exception to the rule, for two reasons. First of all, as we have seen, they are the only elements that fill the d orbitals.

Secondly, they are the only elements whose outer-shell electrons are on two different principal energy levels. The orbital filling patterns of transition metals can be identified thus: ns (n-1)d, where n is the number of the period on which the element is located. For instance, zinc, on period 4, has an outer shell of 4s ²3d ¹⁰. In other words, it has two electrons on principal energy level 4, sublevel 4s —as it "should." But it also has 10 electrons on principal energy level 3, sublevel 3d. In effect, then, the transition metals "go back" and fill in the d orbitals of the preceding period.

There are further complications in the patterns of orbital filling for the transition metals, which will only be discussed in passing here as a further explanation as to why these elements are not "representative." Most of them have two s valence electrons, but many have one, and palladium has zero. Nor do they add their d electrons in regular patterns. Moving across period 4, for instance, the number of electrons in the d orbital "should" increase in increments of 1, from 1 to 10. Instead the pattern goes like this: 1, 2, 3, 5, 5, 6, 7, 8, 10, 10.

LANTHANIDES AND ACTINIDES.

The lanthanides and actinides are set apart even further from the transition metals. In most versions of the periodic table, lanthanum (57) is followed by hafnium (72) in the transition metals section of the chart. Similarly, actinium (89) is followed by rutherfordium (104). The "missing" metals—lanthanides and actinides respectively—are listed at the bottom of the chart. There are reasons for this, as well as for the names of these groups.

After the 6s orbital fills with the representative element barium (56), lanthanum does what a transition metal does—it begins filling the 5d orbital. But after lanthanum, something strange happens: cerium (58) quits filling 5d, and moves to fill the 4f orbital. The filling of that orbital continues throughout the entire lanthanide series, all the way to lutetium (71). Thus lanthanides can be defined as those metals that fill the 4f orbital; however, because lanthanum exhibits similar properties, it is usually included with the lanthanides.

A similar pattern occurs for the actinides. The 7s orbital fills with radium (88), after which actinium (89) begins filling the 6d orbital. Next comes thorium, first of the actinides, which begins the filling of the 5f orbital. This is completed with element 103, lawrencium. Actinides can thus be defined as those metals that fill the 5f orbital; but again, because actinium exhibits similar properties, it is usually included with the actinides.

REAL-LIFE APPLICATIONS

Survey of the Transition Metals

The fact that the transition elements are all metals means that they are lustrous or shiny in appearance, and malleable, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons.

Generally speaking, metals are hard, though a few of the transition metals—as well as members of other metal families—are so soft they can be cut with a knife. Like almost all metals, they tend to have fairly high melting points, and extremely high boiling points.

Many of the transition metals, particularly those on periods 4, 5, and 6, form useful alloys—mixtures containing more than one metal—with one another, and with other elements. Because of their differences in electron configuration, however, they do not always combine in the same ways, even within an element. Iron, for instance, sometimes releases two electrons in chemical bonding, and at other times three.

ABUNDANCE OF THE TRANSITION METALS.

Iron is the fourth most abundant element on Earth, accounting for 4.71% of the elemental mass in the planet's crust. Titanium ranks 10th, with 0.58%, and manganese 13th, with 0.09%. Several other transition metals are comparatively abundant: even gold is much more abundant than many other elements on the periodic table. However, given the fact that only 18 elements account for 99.51% of Earth's crust, the percentages for elements outside of the top 18 tend to very small.

In the human body, iron is the 12th most abundant element, constituting 0.004% of the body's mass. Zinc follows it, at 13th place, accounting for 0.003%. Again, these percentages may not seem particularly high, but in view of the fact that three elements—oxygen, carbon, and hydrogen—account for 93% of human elemental body mass, there is not much room for the other 10 most common elements in the body. Transition metals such as copper are present in trace quantities within the body as well.

DIVIDING THE TRANSITION METALS INTO GROUPS.

There is no easy way to group the transition metals, though certain of these elements are traditionally categorized together. These do not constitute "families" as such, but they do provide useful ways to break down the otherwise rather daunting 40-element lineup of the transition metals.

In two cases, there is at least a relation between group number on the periodic table and the categories loosely assigned to a collection of transition metals. Thus the "coinage metals"—copper, silver, and gold—all occupy Group 9 on the periodic table. These have traditionally been associated with one another because their resistance to oxidation, combined with their malleability and beauty, has made them useful materials for fashioning coins.

Likewise the members of the "zinc group"—zinc, cadmium, and mercury—occupy Group 10 on the periodic table. These, too, have often been associated as a miniature unit due to common properties. Members of the "platinum group"—platinum, iridium, osmium, palladium, rhodium, and ruthenium—occupy a rectangle on the table, corresponding to periods 5 and 6, and groups 6 through 8. What actually makes them a "group," however, is the fact that they tend to appear together in nature.

Iron, nickel, and cobalt, found alongside one another on Period 4, may be grouped together because they are all magnetic to some degree or another. This is far from the only notable characteristic about such metals, but provides a convenient means of further dividing the transition metals into smaller sections.

To the left of iron on the periodic table is a rectangle corresponding to periods 4 through 6, groups 4 through 7. These 11 elements—titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, and rhenium—are referred to here as "alloy metals." This is not a traditional designation, but it is nonetheless useful for describing these metals, most of which form important alloys with iron and other elements.

One element was left out of the "rectangle" described in the preceding paragraph. This is technetium, which apparently does not occur in nature. It is lumped in with a final category, "rare and artificial elements."

It should be stressed that there is nothing hard and fast about these categories. The "alloy metals" are not the only ones that form alloys; nickel is used in coins, though it is not called a coinage metal; and platinum could be listed with gold and silver as "precious metals." Nonetheless, the categories used here seem to provide the most workable means of approaching the many transition metals.

Coinage Metals

GOLD.

Gold almost needs no introduction: virtually everyone knows of its value, and history is full of stories about people who killed or died for this precious metal. Part of its value springs from its rarity in comparison to, say iron: gold is present on Earth's crust at a level of about 5 parts per billion (ppb). Yet as noted earlier, it is more abundant than some metals. Furthermore, due to the fact that it is highly unreactive (reactivity refers to the tendency for bonds between atoms or molecules to be made or broken in such a way that materials are transformed), it tends to be easily separated from other elements.

This helps to explain the fact that gold may well have been the first element ever discovered. No ancient metallurgist needed a laboratory in which to separate gold; indeed, because it so often keeps to itself, it is called a "noble" metal—meaning, in this context, "set apart." Another characteristic of gold that made it valuable was its great malleability. In fact, gold is the most malleable of all metals: A single troy ounce (31.1 g) can be hammered into a sheet just 0.00025 in (0.00064 cm) thick, covering 68 ft² (6.3 m²).

Gold is one of the few metals that is not silver, gray, or white, and its beautifully distinctive color caught the eyes of metalsmiths and royalty from the beginning of civilization. Records from India dating back to 5000 b.c. suggest a familiarity with gold, and jewelry found in Egyptian tombs indicates the use of sophisticated techniques among the goldsmiths of Egypt as early as 2600 b.c. Likewise the Bible mentions gold in several passages. The Romans called it aurum ("shining dawn"), which explains its chemical symbol, Au.

Early chemistry had its roots among the medieval alchemists, who sought in vain to turn plain metals into gold. Likewise the history of European conquest in the New World is filled with bitter tales of conquistadors, lured by gold, who wiped out entire nations of native peoples.

At one time, gold was used in coins, and nations gauged the value of their currency in terms of the gold reserves they possessed. Few coins today—with special exceptions, such as the South African Krugerrand—are gold. Likewise the United States, for instance, went off the gold standard (that is, ceased to tie its currency to gold reserves) in the 1930s; nonetheless, the federal government maintains a vast supply of gold bullion at Fort Knox in Kentucky.

Gold is as popular as ever for jewelry and other decorative objects, of course, but for the most part, it is too soft to have many other commercial purposes. One of the few applications for gold, a good conductor of electricity, is in some electronic components. Also, the radioactive gold-198 isotope is sometimes implanted in tissues as a means of treating forms of cancer.

SILVER.

Like gold, silver has been a part of human life from earliest history. Usually it is considered less valuable, though some societies have actually placed a higher value on silver because it is harder and more durable than gold. In the seventh century b.c., the Lydian civilization of Asia Minor (now Turkey) created the first coins using silver, and in the sixth century b.c., the Chinese began making silver coins. Succeeding dynasties in China continued to mint these coins, round with square holes in them, until the early twentieth century.

The Romans called silver argentum, and therefore today its chemical symbol is Ag. Its uses are much more varied than those of gold, both because of its durability and the fact that it is less expensive. Alloyed with copper, which adds strength to it, it makes sterling silver, used in coins, silverware, and jewelry. Silver nitrate compounds are used in silver plating, applied in mirrors and tableware. (Most mirrors today, however, use aluminum.)

Like gold, silver was once widely used for coinage, and silver coins are still more common than their gold counterparts. In 1963, however, the United States withdrew its silver certifications, paper currency exchangeable for silver. Today, silver coins are generally issued in special situations, such as to commemorate an event.

A large portion of the world's silver supply is used by photographers for developing pictures. In addition, because it is an excellent conductor of heat and electricity, silver has applications in the electronics industry; however, its expense has led many manufacturers to use copper or aluminum instead. Silver is also present, along with zinc and cadmium, in cadmium batteries. Like gold, though to a much lesser extent, it is still an important jewelry-making component.

COPPER.

Most people think of pennies as containing copper, but in fact the penny is the only American coin that contains no copper alloys. Because the amount of copper necessary to make a penny today costs more than $0.01, a penny is actually made of zinc with a thin copper coating. Yet copper has long been a commonly used coinage metal, and long before that, humans used it for other purposes.

Seven thousand years ago, the peoples of the Tigris-Euphrates river valleys, in what is now Iraq, were mining and using copper, and later civilizations combined copper with zinc to make bronze. Indeed, the history of prehistoric and ancient humans' technological development is often divided according to the tools they made, the latter two of which came from transition metals: the Stone Age, the Bronze Age (c. 3300-1200 b.c.), and the Iron Age.

Thus, by the time the Romans dubbed copper cuprum (from whence its chemical symbol, Cu, comes), copper in various forms had long been in use. Today copper is purified by a form of electrolysis, in which impurities such as iron, nickel, arsenic, and zinc are removed. Other "impurities" often present in copper ore include gold, silver, and platinum, and the separation of these from the copper pays for the large amounts of electricity used in the electrolytic process.

Like the other coinage metals, copper is an extremely efficient conductor of heat and electricity, and because it is much less expensive than the other two, pure copper is widely used for electrical wiring. Because of its ability to conduct heat, copper is also applied in materials used for making heaters, as well as for cookware. Due to the high conductivity of copper, a heated copper pan has a uniform temperature, but copper pots must be coated with tin because too much copper in food is toxic.

Copper is also like its two close relatives in that it resists corrosion, and this makes it ideal for plumbing. Its use in making coins resulted from its anti-corrosive qualities, combined with its beauty: like gold, copper has a distinctive color. This aesthetic quality led to the use of copper in decorative applications as well: many old buildings used copper roofs, and the Statue of Liberty is covered in 300 thick copper plates.

Why, then, is the famous statue not copper-colored? Because copper does eventually corrode when exposed to air for long periods of time. Over time, it develops a thin layer of black copper oxide, and as the years pass, carbon dioxide in the air leads to the formation of copper carbonate, which imparts a greenish color.

The human body is about 0.0004% copper, though as noted, larger quantities of copper can be toxic. Copper is found in foods such as shell-fish, nuts, raisins, and dried beans. Whereas human blood has hemoglobin, a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper performs a similar function.

The Zinc Group

ZINC.

Together with copper, zinc appeared in another alloy that, like bronze, helped define the ancient world: brass. (The latter is mentioned in the Bible, for instance in the Book of Daniel, when King Nebuchadnezzar dreams of a statue containing brass and other substances, symbolizing various empires.) Used at least from the first millennium b.c. onward, brass appeared in coins and ornaments throughout Asia Minor. Though it is said that the Chinese purified zinc in about a.d. 1000, the Swiss alchemist Paracelsus (1493-1541) is usually credited with first describing zinc as a metal.

Bluish-white, with a lustrous sheen, zinc is found primarily in the ore sulfide sphalerite. The largest natural deposits of zinc are in Australia and the United States, and after mining, the metal is subjected to a purification and reduction process involving carbon. Zinc is used in galvanized steel, developed in the eighteenth century by Italian physicist Luigi Galvani (1737-1798).

Just as a penny is not really copper but zinc, "tin" roofs are usually made of galvanized steel. Highly resistant to corrosion, galvanized steel finds application in everything from industrial equipment to garbage cans. Zinc oxide is applied in textiles, batteries, paints, and rubber products, while luminous zinc sulfide appears in television screens, movie screens, clock dials, and fluorescent light bulbs.

Zinc phosphide is used as a rodent poison. Like several other transition metals, zinc is a part of many living things, yet it can be toxic in large quantities or specific compounds. For a human being, inhaling zinc oxide causes involuntary shaking. On the other hand, humans and many animals require zinc in their diets for the digestion of proteins. Furthermore, it is believed that zinc contributes to the healing of wounds and to the storage of insulin in the pancreas.

CADMIUM.

In 1817, German chemist Friedrich Strohmeyer (1776-1835) was working as an inspector of pharmacies for the German state of Hanover. While making his rounds, he discovered that one pharmacy had a sample of zinc carbonate labeled as zinc oxide, and while inspecting the chemical in his laboratory, he discovered something unusual. If indeed it were zinc carbonate, it should turn into zinc oxide when heated, and since both compounds were white, there should be no difference in color. Instead, the mysterious compound turned a yellowish-orange.

Strohmeyer continued to analyze the sample, and eventually realized that he had discovered a new element, which he named after the old Greek term for zinc carbonate, kadmeia. Indeed, cadmium typically appears in nature along with zinc or zinc compounds. Silvery white and lustrous or shiny, cadmium is soft enough to be cut with a knife, but chemically it behaves much like zinc: hence the idea of a "zinc group."

Today cadmium is used in batteries, and for electroplating of other metals to protect them against corrosion. Because the cost of cadmium is high due to the difficulty of separating it from zinc, cadmium electroplating is applied only in specialized situations. Cadmium also appears in the control rods of nuclear power plants, where its ready absorption of neutrons aids in controlling the rate at which nuclear fission occurs.

Cadmium is highly toxic, and is believed to be the cause behind the outbreak of itai-itai ("ouch-ouch") disease in Japan in 1955. People ingested rice oil contaminated with cadmium, and experienced a number of painful side effects associated with cadmium poisoning: nausea, vomiting, choking, diarrhea, abdominal pain, headaches, and difficulty breathing.

MERCURY.

One of only two elements—along with bromine—that appears in liquid form at room temperature, mercury is both toxic and highly useful. The Romans called it hydragyrum ("liquid silver"), from whence comes its chemical symbol, Hg. Today, however, it is known by the name of the Romans' god Mercury, the nimble and speedy messenger of the gods. Mercury comes primarily from a red ore called cinnabar, and since it often appears in shiny globules that form outcroppings from the cinnabar, it was relatively easy to discover.

Several things are distinctive about mercury, including its bright silvery color. But nothing distinguishes it as much as its physical properties—not only its liquidity, but the fact that it rolls rapidly, like the fleet-footed god after which it is named. Its surface tension (the quality that causes it to bead) is six times greater than that of water, and for this reason, mercury never wets the surfaces with which it comes in contact.

Mercury, of course, is widely used in thermometers, an application for which it is extremely well-suited. In particular, it expands at a uniform rate when heated, and thus a mercury thermometer (unlike earlier instruments, which used water, wine, or alcohol) can be easily calibrated. (Note that due to the toxicity of the element, mercury thermometers in schools are being replaced by other types of thermometers.) At temperatures close to absolute zero, mercury loses its resistance to the flow of electric current, and therefore it presents a promising area of research with regard to superconductivity.

Even with elements present in the human body, such as zinc, there is a danger of toxicity with exposure to large quantities. Mercury, on the other hand, does not occur naturally in the human body, and is therefore extremely dangerous. Because it is difficult for the body to eliminate, even small quantities can accumulate and exert their poisonous effects, resulting in disorders of the nervous system. In fact, the phrase "mad as a hatter" refers to the symptoms of mercury poisoning suffered by hatmakers of the nineteenth century, who used a mercury compound in making hats of beaver and fur.

Magnetic Metals

IRON.

In its purest form, iron is relatively soft and slightly magnetic, but when hardened, it becomes much more so. As with several of the elements discovered long ago, iron has a chemical symbol (Fe) reflecting an ancient name, the Latin ferrum. But long before the Romans' ancestors arrived in Italy, the Hittites of Asia Minor were purifying iron ore by heating it with charcoal over a hot flame.

The Hittites jealously guarded their secret, which gave them a technological advantage over enemies such as the Egyptians. But when Hittite civilization crumbled in the wake of an invasion by the mysterious "Sea Peoples" in about 1200 b.c., knowledge of ore smelting spread throughout the ancient world. Thus began the Iron Age, in which ancient technology matured greatly. (It should be noted that the Bantu peoples, in what is now Nigeria and neighboring countries, developed their own ironmaking techniques a few centuries later, apparently without benefit of contact with any civilization exposed to Hittite techniques.)

Ever since ancient times, iron has been a vital part of human existence, and with the growth of industrialization during the eighteenth century, demand for iron only increased. But heating iron ore with charcoal required the cutting of trees, and by the latter part of that century, England's forests had been so badly denuded that British ironmakers sought a new means of smelting the ore. It was then that British inventor Abraham Darby (1678?-1717) developed a method for making coke from soft coal, which was abundant throughout England. Adoption of Darby's technique sped up the rate of industrialization in England, and thus advanced the entire Industrial Revolution soon to sweep the Western world.

Symbolic of industrialism was iron in its many forms: pig iron, ore smelted in a coke-burning blast furnace; cast iron, a variety of mixtures containing carbon and/or silicon; wrought iron, which contains small amounts of various other elements, such as nickel, cobalt, copper, chromium, and molybdenum; and most of all steel. Steel is a mixture of iron with manganese and chromium, purified with a blast of hot air in the Bessemer converter, named after English inventor Henry Bessemer (1813-1898). Modern techniques of steelmaking improved on Bessemer's design by using pure oxygen rather than merely hot air.

The ways in which iron is used are almost too obvious (and too numerous) to mention. If iron and steel suddenly ceased to exist, there could be no skyscrapers, no wide-span bridges, no ocean liners or trains or heavy machinery or automobile frames. Furthermore, alloys of steel with other transition metals, such as tungsten and niobium, possess exceptionally great strength, and find application in everything from hand tools to nuclear reactors. Then, of course, there are magnets and electromagnets, which can only be made of iron and/or one of the other magnetic elements, cobalt and nickel.

In the human body, iron is a key part of hemoglobin, the molecule in blood that transports oxygen from the lungs to the cells. If a person fails to get sufficient quantities of iron—present in foods such as red meat and spinach—the result is anemia, characterized by a loss of skin color, weakness, fainting, and heart palpitations. Plants, too, need iron, and without the appropriate amounts are likely to lose their color, weaken, and die.

COBALT.

Isolated in about 1735 by Swedish chemist Georg Brandt (1694-1768), cobalt was the first metal discovered since prehistoric, or at least ancient, times. The name comes from Kobald, German for "underground gnome," and this reflects much about the early history of cobalt. In legend, the Kobalden were mischievous sprites who caused trouble for miners, and in real life, ores containing the element that came to be known as cobalt likewise caused trouble to men working in mines. Not only did these ores contain arsenic, which made miners ill, but because cobalt had no apparent value, it only interfered with their work of extracting other minerals.

Yet cobalt had been in use by artisans long before Brandt's isolated the element. The color of certain cobalt compounds is a brilliant, shocking blue, and this made it popular for the coloring of pottery, glass, and tile. The element, which makes up less than 0.002% of Earth's crust, is found today primarily in ores extracted from mines inCanada, Zaire, and Morocco. One of the most important uses of cobalt is in a highly magnetic alloy known as alnico, which also contains iron, nickel, and aluminum. Combined with tungsten and chromium, cobalt makes stellite, a very hard alloy used in drill bits. Cobalt is also applied in jet engines and turbines.

NICKEL.

Moderately magnetic in its pure form, nickel had an early history much like that of cobalt. English workers mining copper were often dismayed to find a metal that looked like copper, but was not, and they called it "Old Nick's copper"—meaning that it was a trick played on them by Old Nick, or the devil. The Germans gave it a similar name: Kupfernickel, or "imp copper."

Though nickel was not identified as a separate metal by Swedish mineralogist Axel Fredrik Cronstedt (1722-1765) until the eighteenth century, alloys of copper, silver, and nickel had been used as coins even in ancient Egypt. Today, nickel is applied, not surprisingly, in the American five-cent piece—that is, the "nickel"—made from an alloy of nickel and copper. Its anti-corrosive nature also provides a number of other applications for nickel: alloyed with steel, for instance, it makes a protective layer for other metals.

The Platinum Group

PLATINUM.

First identified by an Italian physician visiting the New World in the mid-sixteenth century, platinum—now recognized as a precious metal—was once considered a nuisance in the same way that nickel and cadmium were. Miners, annoyed with the fact that it got in the way when they were looking for gold, called it platina, or "little silver." One of the reasons why platinum did not immediately catch the world's fancy is because it is difficult to extract, and typically appears with the other metals of the "platinum group": iridium, osmium, palladium, rhodium, and ruthenium.

Only in 1803 did English physician and chemist William Hyde Wollaston (1766-1828) develop a means of extracting platinum, and when he did, he discovered that the metal could be hammered into all kinds of shapes. Platinum proved such a success that it made Wollaston financially independent, and he retired from his medical practice at age 34 to pursue scientific research. Today, platinum is used in everything from thermometers to parts for rocket engines, both of which take advantage of its ability to with stand high temperatures.

IRIDIUM AND OSMIUM.

In the same year that Wollaston isolated platinum, three French chemists applied a method of extracting it with a mixture of nitric and hydrochloric acids. The process left a black powder that they were convinced was a new element, and in 1804 English chemist Smithson Tennant (1761-1815) gave it the name iridium. Iris was the Greek goddess of the rainbow, and the name reflected the element's brilliant, multi-colored sheen. As with platinum, iridium is used today in a number of ways, particularly for equipment such as laser crystals that must with stand high temperatures. (In addition, one particular platinumiridium bar provides the standard measure of a kilogram.)

Tennant discovered a second element in 1804, also from the residue left over from the acid process for extracting platinum. This one had a distinctive smell when heated, so he named it osmium after osme, Greek for "odor." In 1898, Austrian chemist Karl Auer, Baron von Welsbach (1858-1929), developed a light bulb using osmium as a filament, the material that is heated. Though osmium proved too expensive for commercial use, Auer's creation paved the way for the use of another transition metal, tungsten, in making long-lasting filaments. Osmium, which is very hard and resistant to wear, is also used in electrical devices, fountain-pen tips, and phonograph needles.

PALLADIUM, RHODIUM, AND RUTHENIUM.

By the time Tennant isolated osmium, Wollaston had found yet another element that emerged from the refining of platinum with nitric and hydrochloric acids. This was palladium, named after a recently discovered asteroid, Pallas. Soft and malleable, palladium resists tarnishing, which makes it valuable to the jewelry industry. Combined with yellow gold, it makes the alloy known as white gold. Because palladium has the unusual ability of absorbing up to 900 times its volume in hydrogen, it provides a useful means of extracting that element.

Also in 1805, Wollaston discovered rhodium, which he named because it had a slightly red color. The density of rhodium is lower, and the melting point higher, than for most of the platinum group elements, and it is often used as an alloy to harden platinum. As with many elements in this group, it has a number of high-temperature applications. When electroplated, it is highly reflective, and this makes it useful as a material for optical instruments.

The element that came to be known as ruthenium had been detected several times before 1844, when Russian chemist Carl Ernest Claus (1796-1864) finally identified it as a distinct element. Thus it was the only platinum group element in whose isolation neither Tennant nor Wollaston played a leading role. Since it had been found in Russia, the element was called ruthenium, in honor of that country's ancient name, Ruthenia.

Alloy Metals

The elements described here as "alloy metals" will be treated briefly, but the amount of space given to them here does not reflect their importance to civilization. Several of these are relatively abundant, and many—titanium, tungsten, manganese, chromium and others—are vital parts of daily life. However, due to space considerations, the treatment of these metals that follows is a regrettably abbreviated one.

As suggested by the informal name given to them here, they are often applied in alloys, typically with other transition metals such as iron, or with other "alloy metals." They are also used in a variety of compounds for a wide variety of applications.

Titanium, Zirconium, and Hafnium

English clergyman and amateur scientist William Gregor (1761-1817) in 1791 noticed what he called "a new metallic substance" in a mineral called menchanite, which he found in the Menachan region of Cornwall, England. Four years later, German chemist Martin Heinrich Klaproth (1743-1817) studied menchanite and found that it contained a new element, which he named titanium after the Titans of Greek mythology. Only in 1910, however, was the element isolated. Due to its high strength-to-weight ratio, titanium is applied today in alloys used for making airplanes and spacecraft. It is also used in white paints and in a variety of other combinations, mostly in alloys containing other transition metals, as well as in compounds with nonmetals such as oxygen.

Six years before he identified titanium, in 1789, Klaproth had found another element, which he named zirconium after the mineral zircon in which it was found. Zircon had been in use for many centuries, and another zirconium-containing mineral, jacinth, is mentioned in the Bible. As with titanium, there was a long period from identification to isolation, which did not occur until 1914. Zirconium alloys are used today in a number of applications, including superconducting magnets and as a construction metal in nuclear power plants. Compounds including zirconium also have a variety of uses, ranging from furnace linings to poison ivy lotion.

Because it is almost always found with zirconium, hafnium—named after the ancient name of Copenhagen, Denmark—proved extremely difficult to isolate. Only in 1923 was it isolated, using x-ray analysis, and today it is applied primarily in light bulb filaments and electrodes. Sometimes it is accidentally "applied," when zirconium is actually the desired metal, because the two are so difficult to separate.

VANADIUM, TANTALUM, AND NIOBIUM.

Vanadium, named after the Norse goddess Vanadis or Freyja, was first identified in 1801, but only isolated in 1867. It is often applied in alloys with steel, and sometimes used for bonding steel and titanium.

Tantalum and niobium are likewise named after figures from mythology—in this case, Greek. These two elements appear together so often that their identification as separate substances was a long process, involving numerous scientists and heated debates. For this reason, alloys involving tantalum—valued for its high melting point—usually include niobium as well.

CHROMIUM, TUNGSTEN, AND MANGANESE, AND OTHER METALS.

Because it displays a wide variety of colors, chromium was named after the Greek word for "color." It is used in chrome and tinted glass, and as a strengthening agent for stainless steel. Tungsten likewise strengthens steel, in large part because it has the highest melting point of any metal: 6,170°F (3,410°C). Its name comes from a Swedish word meaning "heavy stone," and its chemical symbol (W) refers to the ore named wolframite, in which it was first discovered. Its high resistance to heat gives tungsten alloys applications in areas ranging from light bulb filaments to furnaces to missiles.

Likewise manganese, first identified in the mid-eighteenth century, is extraordinarily strong. Its major use is in steelmaking. In addition, compounds such as manganese oxide are used in fertilizers. Other alloy metals include rhenium (identified only in 1925) and molybdenum. These two are often applied in alloys together, and with tungsten, for a variety of industrial purposes.

Rare and Artificial Elements

RARE EARTH-LIKE ELEMENTS.

Though they are not part of the lanthanide series, scandium and yttrium share many properties with those elements, once known as the "rare earths." (In this context, "rarity" refers not so much to scarcity as to difficulty of extraction.) Furthermore, like most of the lanthanides, these two elements were first discovered in Scandinavia.

Their origin is reflected in the name given by Swedish chemist Lars Nilson (1840-1899) to scandium, which he discovered in 1876. As for yttrium, it came from the complex mineral known as ytterite, from whence emerged many of the lanthanides. Yttrium is used in alloys to impart strength, and has a wide if specialized array of applications. Scandium has no important uses.

ARTIFICIAL ELEMENTS.

Technetium, with an atomic number of 43, is unusual in that it is one of the few elements with an atomic number less than 92 that does not occur in nature. This is reflected in its name, from the Greek technetos, meaning "artificial." The strange thing is that technetium, produced in a laboratory in 1936, does occur naturally in the universe—but it seems to be found only in the spectral lines from older stars, and not in those of younger stars such as our Sun.

The other elements in the transition metals family are referred to as "transuranium elements," meaning that they have atomic numbers higher than that of uranium (92). These have all been created artificially; all are highly radioactive; few survive for more than a few minutes (at most); and few have applications in ordinary life.

WHERE TO LEARN MORE

The Copper Page (Web site). <http://www.copper.org>(May 26, 2001).

Gold Institute (Web site). <http://www.goldinstitute.org>(May 26, 2001).

Kerrod, Robin. Matter and Materials. Illustrated by Terry Hadler. Tarrytown, N.Y.: Benchmark Books, 1996.

Mebane, Robert C. and Thomas R. Rybolt. Metals. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995.

Oxlade, Chris. Metal. Chicago, IL: Heinemann Library, 2001.

"The Pictorial Periodic Table" (Web site). <http://chemlab.pc.maricopa.edu/periodic/periodic.html> (May 22, 2001).

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998.

"Transition Metals" ChemicalElements.com (Web site). <http://www.chemicalelements.com/groups/transition.html> (May 26, 2001).

"The Transition Metals." University of Colorado Department of Physics (Web site). <http://www.colorado.edu/physics/2000/periodic_table/transition_elements.html> (May 26, 2001).

WebElements (Web site). <http://www.webelements.com> (May 22, 2001).

Zincworld (Web site). <http://www.zincworld.org> (May22, 2001).

KEY TERMS

ACTINIDES:

Those transition metalsthat fill the 5f orbital. Because actinium—which does not fill the 5f orbital—exhibits characteristics similar to those of the actinides, it is usually considered part of the actinides family.

ALLOY:

A mixture containing more than one metal.

ELECTROLYSIS:

The use of an electric current to cause a chemical reaction.

ELECTRON CLOUD:

A term used to describe the pattern formed by orbitals.

GROUPS:

Columns on the periodic table of elements. These are ordered according to the numbers of valence electrons in the outer shells of the atoms for the elements represented.

INNER TRANSITION METALS:

The lanthanides and actinides, both of which fill the f orbitals. For this reason, they are usually treated separately.

ION:

An atom or group of atoms that has lost or gained one or more electrons, and thus has a net electric charge.

ISOTOPES:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. Isotopes may be either stable or unstable. The latter type, known as radioisotopes, are radioactive.

IUPAC SYSTEM:

A version of the periodic table of elements, authorized by the International Union of Pure and Applied Chemistry (IUPAC), which numbers all groups on the table from 1-18. Thus in the IUPAC system, in use primarily outside of North America, both the representative or main-group elements and the transition metals are numbered.

LANTHANIDES:

The transition metalsthat fill the 4f orbital. Because lanthanum—which does not fill the 4f orbital—exhibits characteristics similar to those of the lanthanides, it is usually considered part of the lanthanide family.

NORTH AMERICAN SYSTEM:

A version of the periodic table of elements that only numbers groups of elements in which the number of valence electrons equals the group number. Hence the transition metalsare usually not numbered in this system. Some North American charts, however, do provide group numbers for transition metals by including an "A" after the groupnumbers for representative or main-group elements, and "B" after those of transition metals.

ORBITAL:

A pattern of probabilities indicating the regions that may be occupied by an electron. The higher the principal energy level, the more complex the pattern of orbitals.

PERIODIC TABLE OF ELEMENTS:

A chart that shows the elements arranged in order of atomic number, along with chemical symbol and the average atomic mass (in atomic mass units) for that particular element. Elements are arranged in groups or columns, as well as in rows or periods, according to specific aspects of their valence electron configurations.

PERIODS:

Rows on the periodic table of elements. These represent successive principal energy levels for the valence electrons in the atoms of the elements involved. Elements in the transition metal family occupy periods 4, 5, 6, or 7.

PRINCIPAL ENERGY LEVEL:

A value indicating the distance that an electron may move away from the nucleus of anatom. This is designated by a whole-number integer, beginning with 1 and moving upward. The higher the principal energy level, the greater the energy in the atom, and the more complex the pattern of orbitals. Elements in the transition metal family have principal energy levels of 4, 5,6, or 7.

RADIOACTIVITY:

A term describing a phenomenon whereby certain isotopes known as radioisotopes are subject to a form of decay brought about by the emission of high-energy particles. "Decay" does not mean that the isotope "rots"; rather, it decays to form another isotope until eventually (though this may take a long time), it becomes stable.

REACTIVITY:

The tendency for bonds between atoms or molecules to be made or broken in such a way that materials aretransformed.

REPRESENTATIVE OR MAIN-GROUPELEMENTS:

The 44 elements in Groups 1 through 8 on the periodic table of elements in the North American system, for which the number of valence electrons equals the group number. (The only exception is helium.) In the IUPAC system, these elements are assigned group numbers 1, 2, and 13 through 18. (In these last six columns on the IUPAC chart, there is no relation between the number of valence electrons and group number.)

SHELL:

The orbital pattern of the valence electrons at the outside of an atom.

SUBLEVEL:

A region within the principal energy level occupied by electrons in anatom. Whatever the number n of the principal energy level, there are n sublevels. At each principal energy level, the first sublevel to be filled is the one corresponding to the s orbital pattern, followed by the p and then the d pattern.

TRANSITION METALS:

A group of 40 elements, which are not assigned a group number in the version of the periodic table of elements known as the North Americansystem. These are the only elements that fill the d orbital. In addition, the transition metals—unlike the representative or main-group elements—have their valence electrons on two different principal energy levels. Though the lanthanides and actinides are considered inner transition metals, they are usually treated separately.