HAFNIUM
SYMBOL: Hf PERIOD: 6 GROUP: 4 (IVB) ATOMIC NO: 72
ATOMIC MASS: 178.49 amu VALENCE: 2, 3, and 4 OXIDATION STATE: +4 NATURAL STATE: Solid
ORIGIN OF NAME: Named after Hafnia, the Latin name for the city of Copenhagen, Den- mark.
ISOTOPES: There are 44 known isotopes for hafnium. Five are stable and one of the unsta-
ble isotopes has such a long half-life (Hf-174 with a 2.0×10+15 years) that it is included
as contributing 0.16% to the amount of hafnium found in the Earth’s crust. The percent- age contributions of the 5 stable isotopes to the element’s natural existence on Earth are as follows: Hf-176 = 5.26%, Hf-177 = 18.60%, Hf-178 = 27.28%, Hf-179 = 13.62%, and Hf-180 = 35.08%.
ELECTRON CONFIGURATION
Energy Levels/Shells/Electrons Orbitals/Electrons
1-K = 2 s2 2-L = 8 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
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Properties
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/cm3. 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 (72Hf) 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 (57La to 71Li). 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.
Abundance and Source
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 (ZrO2), a zirconium ore, and zircon (ZrSiO4) 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
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.
Common Uses
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.
Examples of Compounds
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 (HfO2): 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: Hf4+ + 4Cl → HfCl
4, also known as hafnium tetrachloride.
Hafnium fluoride: Hf4+ + 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) ATOMIC NO: 73
ATOMIC MASS: 180.948 amu VALENCE: 2, 3, and 5 OXIDATION STATE: +5 NATU- RAL STATE: Solid
ORIGIN OF NAME: Tantalum was named after Tantalus, who was the father of Niobe, the queen of Thebes, a city in Greek mythology. (Note: The element tantalum was originally confused with the element nobelium.)
ISOTOPES: There are 49 isotopes of tantalum. Only the isotope Ta-181 is stable and accounts for 99.988% of the total mass of the element on Earth. Just 0.012% of the
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ELECTRON CONFIGURATION
Energy Levels/Shells/Electrons Orbitals/Electrons
1-K = 2 s2 2-L = 8 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
element’s mass is contributed by Ta-180, which has a half-life of 1.2×10+15 years and is
thus considered naturally stable. The remaining 47 isotopes are all artificially produced in nuclear reactions or particle accelerators and have half-lives ranging from a few micro- seconds to few days to about two years.
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/cm3.
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.
Abundance and Source
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)2O6]; tantalite [(Fe, Mn)(Ta, Nb)2O6]; and euxenite [(Y, Ca, Er, La, Ce, U, Th)(Nb, Ta, Ti)2O6]. 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%
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.
Common Uses
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 (Ta2O5) 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.
Examples of Compounds
Tantalum pentoxide is representative of tantalum’s stable oxidation state of +5: 2Ta5+ + 5O2-
→ Ta2O5. 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 (TaS2) is used to make solid lubricants and special noncorrosive greases.
Tantalum fluoride (TaF5) is a catalyst used to speed up organic chemical reactions.
Tantalum pentoxide (Ta2O5) 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 inhaled or ingested, but the metal itself is nonpoisonous.
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ELECTRON CONFIGURATION
Energy Levels/Shells/Electrons Orbitals/Electrons
1-K = 2 s2 2-L = 8 s2, p6 3-M = 18 s2, p6, d10 4-N = 32 s2, p6, d10, f14 5-O = 12 s2, p6, d4 6-P = 2 s2 TUNGSTEN
SYMBOL: W PERIOD: 6 GROUP: 6 (VIB) ATOMIC NO: 74
ATOMIC MASS: 183.85 amu VALENCE: 2, 4, 5, and 6 OXIDATION STATE: +4, and +6 NATURAL STATE: Solid
ORIGIN OF NAME:Tungsten was originally named “Wolfram” by German scientists, after
the mineral in which it was found, Wolframite—thus, its symbol “W.”Later, Swedish scien-
tists named it tung sten, which means “heavy stone,” but it retained its original symbol of “W.”
ISOTOPES: There are 36 isotopes of tungsten. Five are naturally stable and therefore con- tribute proportionally to tungsten’s existence on Earth, as follows: W-180 = 0.12%, W- 182 = 26.50%, W-183 = 14.31%, W-184 = 30.64%, and W-186 = 28.43%. The other 31 isotopes are man-made in nuclear reactors and particle accelerators and have half- lives ranging from fractions of a second to many days.
Properties
Extremely pure samples of tungsten are rather soft and can be cut easily with a simple saw. Pure tungsten can be drawn into fine wires (ductile). On the other hand, if there are even a few impurities in the sample, the metal becomes very hard and brittle. It is a very dense metal with a whitish-to-silvery-grayish color when freshly cut. It has the highest melting point of all met- als at 3,422°C, making it a useful metal where high temperatures are required. Incidentally, the transition metals on both sides of it in period 6 (73Ta and 75Re) have the second- and third- highest melting points. Tungsten’s boiling point is also high at 5,927°C.
Characteristics
Tungsten is considered part of the chromium triad of group six (VIB), which consists of 24Cr, 42Mo, and 74W. These elements share many of the same physical and chemical attributes. Tungsten’s high melting point makes it unique insofar as it can be heated to the point that it glows with a very bright white light without melting. This makes it ideal as a filament for incandescent electric light bulbs. Most metals melt long before they reach the point of incandescence.
Chemically, tungsten is rather inert, but it will form compounds with several other ele- ments at high temperatures (e.g., the halogens, carbon, boron, silicon, nitrogen, and oxygen). Tungsten will corrode in seawater.
Abundance and Source
Tungsten is the 58th most abundant element found on Earth. It is never found in 100% pure form in nature. Its major ore is called wolframite or tungsten tetroxide, (Fe,Mn)WO4, which is a mixture of iron and manganese and tungsten oxide. During processing, the ore is pulverized and treated with strong alkalis resulting in tungsten trioxide (WO3), which is then heated (reduced) with carbon to remove the oxygen. This results in a variety of bright color changes and ends up as a rather pure form of tungsten metal: 2WO3 + 3C → 2WO + 3CO2. Or, if hydrogen is used as the reducing agent, a more pure form of metal is produced: WO3 + 3H2→ W + 3H2O.
Tungsten ores (oxides) are found in Russia, China, South America, Thailand, and Canada. In the United States, the ores are found in Texas, New Mexico, Colorado, California, Arizona, and Nebraska. Today, it is estimated that about 75% of all tungsten is found in China.
The tungsten ore called “scheelite” is named after Carl Wilhelm Scheele (1742–1786), who studied and experimented with tungsten minerals, but as with many of his other “near” discoveries, such as oxygen, fluorine, hydrogen sulfide, hydrogen cyanide, and manganese, he was not given credit.
History
In the mid-1700s a number of scientists experimented with and attempted to isolate element 74 by treating ores of other metals with reagents. One problem was that tungsten was often confused with tin and arsenic. It was not until 1783 that Don Fausto de Elhuyar (1755–1833) and his brother Don Juan Jose de Elhuyar isolated a substance from tin ore that they called “wolframite.” They named it after the mineral in which it was found. At about the same time the Swedish named it tung sten, which means “heavy stone” in Swedish. This explains the potentially confusing use of W for the symbol for tungsten.
Common Uses
Since its melting temperature is over 3,400°C, tungsten is one of the few metals that can glow white hot when heated without melting. This factor makes it the second most frequently used industrial metal (the first is iron). Tungsten is used in the filaments of common light bulbs, as well as in TV tubes, cathode ray tubes, and computer monitors. Its ability to be “pulled” into thin wire makes it useful in the electronics industry. It is also used in solar energy products and X-ray equipment. Its ability to withstand high temperatures makes it ideal for rocket engines and electric-heater filaments of all kinds. Tungsten carbide is used as a substitute for diamonds for drills and grinding equipment. This attribute is important in the manufacture of exceptionally hard, high-speed cutting tools.
Examples of Compounds
Most of tungsten’s stable compounds have the main oxidation state of +6 (e.g., W6+ + 6Cl1-
→ WCl6), and the lower oxidation state of +4 occurs in the hard tungsten carbide (e.g., W4+ + C4-→ WC).
Guide to the Elements | 155 Tungsten carbide (WC) is extremely hard and resistant to high temperatures. When cement-
ed to tools, it is as hard as corundum (aluminum oxide) and makes excellent grinding surfaces and cutting edges for machine tools.
Tungsten disulfide (WS2) is used as a solid lubricant that can withstand high temperatures. It is also used as a spray lubricant.
Tungsten oxide (WO3) is used to make tungsten alloys. Tungsten oxide is also used as fire- proofing for various surfaces and is used as a yellow pigment in ceramics.
Tungsten steel is an alloy that acts somewhat like molybdenum to form important steel alloys; tungsten steel is tough and hard, wears well, resists rusting, and will take a sharp cut- ting edge.
Tungsten compounds of calcium and magnesium have phosphorescent properties that make them useful in manufacturing fluorescent lighting fixtures.
Hazards:
Tungsten dust, powder, and fine particles will explode, sometimes spontaneously, in air. The dust of many of tungsten’s compounds is toxic if inhaled or ingested.
RHENIUM
SYMBOL: Re PERIOD: 6 GROUP: 7 (VIIB) ATOMIC NO: 75
ATOMIC MASS: 186.207 amu VALENCE: 4, 6, and 7 OXIDATION STATE: +4. +6, and +7 NATURAL STATE: Solid
ORIGIN OF NAME:Derived from the Latin word Rhenus, which stands for the Rhine River in Western Europe.
ISOTOPES: There are 45 isotopes of rhenium. Only one of these is stable: Re-185, which