Rhenium – facts and properties
- Symbol
- Re
- Atomic number
- 75
- Atomic mass
- 186.207 u
- Period
- 6
- Group
- 7
- Category
- transition metal
- Electron configuration
- [Xe] 4f¹⁴ 5d⁵ 6s²
- Electronegativity
- 1.9
- Atomic radius
- 197 pm
- Valence electrons
- 7
- State at standard conditions
- solid
- Melting point
- 3186 °C
- Boiling point
- 5596 °C
- Density
- 20.8 g/cm³
- Discovered
- 1925
Rhenium is a silvery-white, very heavy transition metal with atomic number 75. It was the last element with a stable isotope to be discovered, and it is among the rarest elements in the Earth's crust. Its name recalls the river Rhine. Rhenium melts only at a very high temperature. In small proportions it keeps the blades of jet engines strong in the heat and extends the life of the platinum catalysts used to make petrol.
Position in the periodic table
Occurrence and extraction
Rhenium is extremely rare. The Earth's crust holds less than a milligram per tonne of rock. There is practically no ore of its own, and only on one volcano in the Kuril Islands has a tiny rhenium mineral been found. The element hides in traces in molybdenum ores, above all molybdenite, and so it is obtained as a by-product of molybdenum and copper production. When the ore is roasted, rhenium heptoxide forms, a volatile oxide that escapes with the smoke and is recovered from the scrubbing liquors of the plants. From it ammonium perrhenate is made, which is reduced with hydrogen to metal powder. Only a few dozen tonnes are produced a year, most of it in Chile.
The element was discovered in 1925 in Berlin by Ida Tacke, Walter Noddack and Otto Berg. They searched ores and minerals by X-ray spectroscopy and found the lines of an unknown element in columbite. They named it after the Rhine, Rhenus in Latin. To prove their discovery, the Noddacks extracted a single gram of the metal from 660 kilograms of molybdenite three years later.
Properties and reactions
Rhenium is in period 6 and group 7, below technetium, which belongs to period 5. Above technetium in turn stands manganese. Its electron configuration is [Xe] 4f¹⁴ 5d⁵ 6s² and its electronegativity is 1.9. The metal has a density of 20.8 g/cm³, melts at 3186 °C and boils at 5596 °C. Among metals only tungsten melts higher, and the boiling point is even slightly above that of tungsten.
Rhenium is hard and yet easy to shape. It keeps its strength even at very high temperatures. At room temperature it is stable in air, and at red heat it oxidises to the volatile heptoxide. Hydrochloric and hydrofluoric acid do not attack the metal, but nitric acid, hot concentrated sulfuric acid and hydrogen peroxide dissolve it.
Remarkable is the variety of oxidation numbers, from minus one to plus seven. The highest state, plus seven, is stable. Perrhenate, ReO₄⁻, is colourless and only a weak oxidising agent, whereas the permanganate of manganese is violet and a strong one. It shows that higher oxidation states become more stable further down a group.
Natural rhenium consists of two isotopes. One of them, rhenium-187, is radioactive but decays so slowly that its half-life is longer than the age of the universe. The radiation is far too weak to be a concern. Geologists use it together with its decay product osmium-187 to determine the age of rocks and meteorites.
Uses in everyday life and technology
Most rhenium goes into nickel-based superalloys. These are cast into the turbine blades of aircraft engines and gas turbines, often as a single crystal without grain boundaries. Just a few percent of rhenium reduce the slow flow of metal under load and heat that is known as creep. The blades can then withstand higher temperatures, and the engine runs more economically.
The second large field is catalysis. In catalytic reforming, raw petrol, called naphtha, is converted into anti-knock fuel with a high octane number. For this, platinum is used together with rhenium on a support of aluminium oxide. With rhenium, less carbon builds up on the catalyst, so it works for longer and the plant can run at lower pressure.
Other applications exploit its heat resistance. Thermocouples made of tungsten-rhenium alloys measure temperatures of well over two thousand degrees. Rhenium ribbons serve as filaments in mass spectrometers, ionisation gauges and flash lamps.
Quick question
What is the atomic number of rhenium (Re)?
FAQ
Why is rhenium called rhenium?
The name goes back to Rhenus, the Latin name of the river Rhine. It was chosen by the discoverers Ida Tacke, Walter Noddack and Otto Berg, who worked in Berlin.
What is rhenium used for?
Mainly for nickel-based superalloys in turbine blades and for platinum-rhenium catalysts that deliver anti-knock petrol in refineries. Smaller amounts go into thermocouples and filaments.
Why is rhenium so rare and expensive?
It forms hardly any minerals of its own and occurs only in traces in other ores. It can be obtained solely as a by-product of molybdenum and copper production. Demand from aviation adds to the price, which is why recycling matters.