RHENIUM: DISCOVERY, PROPERTIES, STRUCTURE, USES - CHEMISTRY - 2025

The rhenium is a metallic element whose chemical symbol is Re, and placed in group 7 of the periodic table, two places below manganese. It shares with this and technetium the property of exhibiting multiple numbers or oxidation states, from +1 to +7. It also forms an anion called perrhenate, ReO ₄ ^-, analogous to permanganate, MnO ₄ ^-.

This metal is one of the rarest and scarces in nature, so its price is high. It is extracted as a by-product of molybdenum and copper mining. One of the most relevant properties of rhenium is its high melting point, barely surpassed by carbon and tungsten, and its high density, being twice that of lead.

Rhenium metal sphere. Source: Hi-Res Images of Chemical Elements / CC BY (https://creativecommons.org/licenses/by/3.0)

His discovery has controversial and unfortunate overtones. The name 'rhenium' derives from the Latin word 'rhenus', which means Rhine, the famous German river near the site where the German chemists who isolated and identified this new element worked.

Rhenium has numerous uses, among which the refinement of the octane number of gasoline stands out, as well as in the manufacture of refractory superalloys, destined for the assembly of turbines and engines of aerospace ships.

Discovery

The existence of two heavy elements with chemical characteristics similar to those of manganese had already been predicted since the years 1869, through the periodic table of the Russian chemist Dmitri Mendeleev. However, it was not known at that time what their atomic numbers should be; and it was here in 1913 that the prediction of the English physicist Henry Moseley was introduced.

According to Moseley, these two elements belonging to the manganese group must have atomic numbers 43 and 75.

A couple of years earlier, however, Japanese chemist Masataka Ogawa had discovered supposed element 43 in a sample of the mineral torianite. After announcing his results in 1908, he wanted to baptize this element with the name 'Niponio'. Unfortunately, chemists at the time proved that Ogawa had not discovered element 43.

And so, other years passed when in 1925 three German chemists: Walter Noddack, Ida Noddack, and Otto Berg, found element 75 in mineral samples of columbite, gadolinite, and molybdenite. These gave him the name of rhenium, in honor of the Rhine river of Germany ('Rhenus', in Latin).

Masataka Ogawa's mistake was to have misidentified the element: he had discovered rhenium, not element 43, which today is called technetium.

Properties of rhenium

Rhenium situation in the periodic table. ! Original: AhoerstemeierVector: Sushant savla / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Physical appearance

Rhenium is usually marketed as a grayish powder. Its metallic pieces, generally spherical drops, are silver-gray, which are also highly shiny.

Molar mass

186.207 g / mol

Atomic number

75

Melting point

3186 ºC

Boiling point

5630 ºC

Density

-At room temperature: 21.02 g / cm ³

-Right at melting point: 18.9 g / cm ³

Rhenium is a metal that is almost twice as dense as lead itself. Thus, a sphere of rhenium weighing 1 gram can be equated to a robust lead crystal of the same mass.

Electronegativity

1.9 on the Pauling scale

Ionization energies

First: 760 kJ / mol

Second: 1260 kJ / mol

Third: 2510 kJ / mol

Molar heat capacity

25.48 J / (mol K)

Thermal conductivity

48.0 W / (m K)

Electrical resistivity

193 nΩ m

Mohs hardness

7

Isotopes

Rhenium atoms occur in nature as two isotopes: ¹⁸⁵ Re, with an abundance of 37.4%; and ¹⁸⁷ Re, with an abundance of 62.6%. Rhenium is one of those elements whose most abundant isotope is radioactive; however, the half-life of ¹⁸⁷ Re is very long (4.12 · 10 ¹⁰ years), so it is practically considered stable.

Reactivity

Rhenium metal is a material resistant to rust. When it does, its oxide, Re ₂ O ₇, volatilizes at high temperatures and burns with a yellowish-green flame. The rhenium pieces resist the attack of concentrated HNO ₃; but when hot, it dissolves to generate rhenic acid and nitrogen dioxide, which turns the solution brown:

Re + 7HNO ₃ → HReO ₄ + 7 NO ₂ + 3H ₂ O

The chemistry of rhenium is vast, as it is capable of forming compounds with a wide spectrum of oxidation numbers, as well as establishing a quadrupole bond between two rhenium atoms (four Re-Re covalent bonds).

Structure and electronic configuration

Electron shell of rhenium. Author: User: GregRobson (Greg Robson). Wikimedia commons

The rhenium atoms group together in their crystals to form a compact hexagonal structure, hcp, which is characterized by being very dense. This is consistent with the fact that it is a high-density metal. The metallic bond, product of the overlap of their external orbitals, keeps the Re atoms strongly cohesive.

In this metallic bond, Re-Re, the valence electrons participate, which are according to the electronic configuration:

4f ¹⁴ 5d ⁵ 6s ²

In principle, it is the 5d and 6s orbitals that overlap to compact the Re atoms in the hcp structure. Note that its electrons add up to a total of 7, corresponding to the number of its group on the periodic table.

Oxidation numbers

The electronic configuration of rhenium allows us to see at once that its atom is capable of losing up to 7 electrons, to become the hypothetical cation Re ⁷⁺. When the existence of Re ⁷⁺ in any rhenium compound is assumed, for example, in Re ₂ O ₇ (Re ₂ ⁷⁺ O ₇ ^2-), it is said to have an oxidation number of +7, Re (VII).

Other positive oxidation numbers for rhenium are: +1 (Re ⁺), +2 (Re ²⁺), +3 (Re ³⁺), and so on up to +7. Likewise, rhenium can gain electrons by becoming an anion. In these cases, it is said to have a negative oxidation number: -3 (Re ^3-), -2 (Re ^2-) and -1 (Re ^-).

Applications

Gasoline

Rhenium, along with platinum, is used to create catalysts that increase the octane rating of gasoline, while lowering its lead content. On the other hand, rhenium catalysts are used for multiple hydrogenation reactions, this due to their resistance to being poisoned by nitrogen, phosphorus and sulfur.

Refractory superalloys

Rhenium is a refractory metal due to its high melting point. That is why it is added to nickel alloys to make them refractory and resistant to high pressures and temperatures. These superalloys are mostly used for the design of turbines and engines for aerospace craft.

Tungsten filaments

Rhenium can also form alloys with tungsten, which improves its ductility and therefore facilitates the manufacture of the filaments. These rhenium-tungsten filaments are used as X-ray sources, and for the design of thermocouples capable of measuring temperatures up to 2200 ºC.

Likewise, these rhenium filaments were once used for the flashes of archaic cameras, and now for the lamps of sophisticated equipment; such as the mass spectrophotometer.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Sarah Pierce. (2020). Rhenium: Uses, History, Facts & Isotopes. Study. Recovered from: study.com
3. National Center for Biotechnology Information. (2020). Rhenium. PubChem Database., CID = 23947. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Wikipedia. (2020). Rhenium. Recovered from: en.wikipedia.org
5. Dr. Doug Stewart. (2020). Rhenium Element Facts. Recovered from: chemicool.com
6. Eric Scerri. (November 18, 2008). Rhenium. Chemistry in its elements. Recovered from: chemistryworld.com