156 | The History and Use of Our Earth’s Chemical Elements Properties
156 | The History and Use of Our Earth’s Chemical Elements Properties
Rhenium ranges in color from silvery-white to gray to a black powder. It is a rather dense element. As a refined metal, rhenium is ductile, but because it is rather rare, its properties have not found many uses. Rhenium does have the widest range of valences. In addition to its com- mon valences of 4, 6, and 7, it also has the uncommon valences of 2, –1, and –7.
Rhenium has a high melting point of 3,180°C, a boiling point of 5,627°C, and a density
of 21.04 g/cm 3 . Characteristics Rhenium is one of the transition elements, which range from metals to metal-like ele-
ments. Its chemical and physical properties are similar to those of technetium, which is above it in the periodic table. It is not very reactive. When small amounts are added to molybdenum, it forms a unique type of semiconducting metal. It is also noncorrosive in seawater.
AbundanceandSource Rhenium is the 78th most common element found on Earth, which makes it somewhat
rare. During the early twentieth century, it required the processing of about a 1,000 pounds of earth to secure just one pound of rhenium, resulting in a price of about $10,000 per gram. Thus, there were few uses for rhenium. Later in the century, improved mining and refining techniques reduced the price. Today, the United States produces about 1,000 pounds of rhe- nium per year, and the world’s total estimated supply is only about 400 tons.
The main sources of rhenium are the molybdenite and columbite ores. Some rhenium is recovered as a by-product of the smelting of copper sulfide (CuS) ores. Molybdenum sulfide (MoS 2 ) is the main ore and is usually associated with igneous rocks and, at times, metallic-like deposits. Molybdenite is found in Chile, as well as in the states of New Mexico, Utah, and Colorado in the United States.
History Rhenium is extremely rare. It is not one of the historic, accidentally discovered elements,
even though there was a “predicted” blank space—atomic number 75 on the periodic table. It was known as one of the manganese group (VIIB). Under the Mendeleevian terminology, element number 43 was known as eka-manganese and element number 75 was called dvi- manganese. The element 75 was specifically sought out by Ida Tacke Noddack (1896–1979), Walter Noddack (1893–1960), and Otto Carl Berg (1875–1939), who calculated, and pre- dicted, some of rhenium’s chemical and physical properties as they searched for number 75. In 1925, by using various analytical techniques, they concentrated some gadolinium ore 100,000 times, which was a Herculean task. This resulted in a small sample that was adequate to study and identify spectroscopically the element 75 that they named after the Rhine River.
An odd property of many elements that helped scientists theoretically determine the char- acteristics of undiscovered elements is that elements having an even number of protons in their nuclei (atomic number) are more commonly found on Earth than are the elements with an odd number of protons. It is unclear why elements with odd numbers of protons in their nuclei are less commonly found than are those with even numbers of protons.
157 CommonUses
Guide to the Elements |
Small quantities of rhenium are alloyed with iron to form steel that is both hard and resis- tant to wear and high-temperatures. Because of its high melting point, rhenium is used in many applications where long-wearing, high-temperature electrical components are required, such as electrical contacts and switches and high-temperature thermocouples. This physical quality makes rhenium alloys ideal for use in rocket and missile engines. It is also used to form the filaments in photographic flash lamps.
Rhenium’s isotope ( 187 Re) has a very long half-life and decays by both beta and alpha radiation at a very steady rate. This factor makes it useful as a standard to measure the age of the universe.
ExamplesofCompounds The +3 and +5 oxidation states of rhenium are demonstrated by reactions with chlorine, as
in, for example, the following reactions:
Rhenium (III) chloride: Re 3+ + 3Cl 1- → ReCl 3 . Rhenium (V) chloride: Re 5+ + 5Cl 1- → ReCl 5 .
The +4 and +6 oxidation states, on the other hand, are associated with fluorine. Some exam- ples follow:
Rhenium (IV) fluoride: Re 4+ + 4F 1- → ReF 4 . Rhenium (VI) fluoride: Re 6+ + 6F 1- → ReF 6 .
Rhenium heptasulfide (Re 2 S 7 ) is one of several possible rhenium sulfur compounds. It is used as a catalyst to speed up chemical reactions. The oxides of rhenium involve the +7 oxidation state of rhenium: Rhenium heptoxide (Re 2 O 7 ), for example, is explosive. There are at least six different forms of rhenium oxides.
Hazards Rhenium is flammable in powder form. Rhenium dust and powder and many of its com-