Rhenium

75 tungsten ← rhenium → osmium Tc ↑ Re ↓ Bh periodic table
General
Name, Symbol, Number	rhenium, Re, 75
Chemical series	transition metals
Group, Period, Block	7, 6, d
Appearance	grayish white
Atomic mass	186.207(1) g/mol
Electron configuration	[Xe] 4f14 5d5 6s2
Electrons per shell	2, 8, 18, 32, 13, 2
Physical properties
Phase	solid
Density (near r.t.)	21.02 g/cm³
Liquid density at m.p.	18.9 g/cm³
Melting point	3459 K (3186 °C, 5767 °F)
Boiling point	5869 K (5596 °C, 10105 °F)
Heat of fusion	60.43 kJ/mol
Heat of vaporization	704 kJ/mol
Heat capacity	(25 °C) 25.48 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 3303 3614 4009 4500 5127 5954
Atomic properties
Crystal structure	hexagonal
Oxidation states	7, 6, 4, 2, −2 (mildly acidic oxide)
Electronegativity	1.9 (Pauling scale)
Ionization energies (more)	1st: 760 kJ/mol
	2nd: 1260 kJ/mol
	3rd: 2510 kJ/mol
Atomic radius	135 pm
Atomic radius (calc.)	188 pm
Covalent radius	159 pm
Miscellaneous
Magnetic ordering	?
Electrical resistivity	(20 °C) 193 nΩ·m
Thermal conductivity	(300 K) 48.0 W/(m·K)
Thermal expansion	(25 °C) 6.2 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 4700 m/s
Speed of sound (thin rod)	(r.t.) 463 m/s
Shear modulus	178 GPa
Bulk modulus	370 GPa
Poisson ratio	0.30
Mohs hardness	7.0
Vickers hardness	2450 MPa
Brinell hardness	1320 MPa
CAS registry number	7440-15-5
Notable isotopes
Main article: Isotopes of rhenium iso NA half-life DM DE (MeV) DP 185Re 37.4% Re is stable with 110 neutrons 187Re 62.6% 4.35×1010 y α 1.653 183Ta β- 0.003 187Os

Rhenium (chemical symbol Re, atomic number 75) is a silvery-white, rare metal that is among the 10 most expensive metals on Earth. It was the last naturally occurring element to be discovered. Chemically, it resembles manganese. It is obtained as a byproduct of molybdenum refinement and rhenium-molybdenum alloys are superconducting.

• heavy metal
• polyvalent transition metal, rhenium resembles manganese chemically and is used in some alloys.

Occurrence

Rhenium is widely spread in the Earth's crust, at approximately 0.001 parts per million (ppm), but it is not found free in nature. Some molybdenum ores contain 0.002% to 0.2% rhenium. It was only recently that the first rhenium mineral was found. In 1994, Nature published a letter describing a rhenium sulfide mineral found condensing from a fumarole on Russia's Kudriavy volcano. This mineral, however, is not an economically viable source of the element.

Commercial rhenium is extracted from molybdenum roaster-flue dusts obtained from copper sulfide ores. Total world production is between 40 and 50 tons/year; the main producers are Chile, the United States, and Kazakhstan. Recycling of used platinum-rhenium catalyst and special alloys allow the recovery of another 10 tons/year.

History

Rhenium (from Latin Rhenus, meaning "Rhine") was the last naturally occurring element to be discovered. The existence of an as-yet-undiscovered element at its position in the periodic table had been predicted by Henry Moseley in 1914. It is generally considered to have been discovered by Walter Noddack, Ida Tacke, and Otto Berg in Germany. In 1925, they reported detecting the element in platinum ore and in the mineral columbite. They also found rhenium in gadolinite and molybdenite. In 1928, they succeeded in extracting 1 gram (g) of the element by processing 660 kilograms (kg) of molybdenite.

The process was so complicated and the cost so high that production was discontinued until early 1950, when tungsten-rhenium and molybdenum-rhenium alloys were prepared. These alloys found important applications in industry, resulting in a high demand for rhenium produced from the molybdenite fraction of porphyry copper ores.

Notable characteristics

Rhenium is a silvery white metal, lustrous, and has one of the highest melting points of all elements, exceeded by only tungsten and carbon. It is also one of the most dense, exceeded only by platinum, iridium, and osmium. The oxidation states of rhenium include -3,-1,+1,+2,+3,+4,+5,+6 and +7 oxidation states. The oxidation states +7,+6,+4,+2 and -1 are the most common.

Its usual commercial form is a powder, but this element can be consolidated by pressing and resistance-sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact shape that is in excess of 90 percent of the density of the metal. When annealed this metal is very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K; tungsten-rhenium alloys are also superconductive, around 4-8 K depending on the alloy [1].

Isotopes

Naturally occurring rhenium is a mix of ¹⁸⁵Re, which is stable, and ¹⁸⁷Re, which is unstable but has a very long half-life. There are twenty-six other unstable isotopes recognized.

Compounds

• Ammonium perrhenate (APR, NH₄ReO₄): This salt of rhenium is the most common form in which rhenium is traded. Its rhenium content is around 69.0–69.4%. Pure rhenium powder can be produced from APR simply by reducing the latter with hydrogen.

• Perrhenic acid (HReO₄): This is a strong acid that can attack metals, metal oxides, carbonates, and hydroxides. It is, however, non-oxidizing. It exists only in solution—the solid has not been isolated. It is a common precursor to other rhenium compounds, some of have applications in the petrochemical and fine chemical industries.

• Rhenium trioxide or rhenium(VI) oxide (ReO₃): This oxide is a red solid with a metallic luster. It is the only stable trioxide of the Group 7 elements (manganese, technetium, rhenium). It can be formed by reducing rhenium(VII) oxide (Re₂O₇) with carbon monoxide. Unlike most oxides, it exhibits very low resistivity. Instead, it behaves more like a metal in that its resistivity decreases as its temperature is lowered. In crystals of the oxide, each rhenium atom is surrounded by six oxygen atoms, forming an ReO₆ octahedron.

Applications

This element is used in platinum-rhenium catalysts which in turn are primarily used in making lead-free, high-octane gasoline and in high-temperature superalloys that are used to make jet engine parts. Other uses:

• Widely used as filaments in mass spectrographs and in ion gauges.
• An additive to tungsten and molybdenum-based alloys to increase ductility in these alloys.
• Rhenium catalysts are very resistant to chemical poisoning, and so are used in certain kinds of hydrogenation reactions.
• Electrical contact material due to its good wear resistance and ability to withstand arc corrosion.
• Thermocouples containing alloys of rhenium and tungsten are used to measure temperatures up to 2200 °C.
• Rhenium wire is used in photoflash lamps in photography.

Precautions

Little is known about the toxicity of rhenium, but as a precautionary measure, it should be handled with care.

See also

• Chemical element
• Metal
• Periodic table
• Transition metal

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory - Rhenium

External links

• WebElements.com - Rhenium
• Chemical assay of ammonium perrhenate from different producers

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