RHODIUM

Name: Rhodium (from “Rhodon,” “pink” in ancient Greek, alluding to the color of its salts)

Symbol: Rh

Group: 9

Period: 5

Block: d

Category: Transition metals

Atomic number: 45

Atomic mass: 102.905 u

Electrons per shell: 2, 8, 18, 16, 1

Electronegativity: 2.28

Density: 12.45 g/cc

Melting point: 1964ºC

Boiling point: 3695ºC

Thermal conductivity: 150 W (m·K)

Electrical conductivity: 2.3 × 10^7 S/m

Magnetic order: Paramagnetic

Ordinary state: Solid

States of Oxidation: -3, -1, 0, 1, 2, 3, 4, 5, 6

Mohs Hardness: 6

Vickers Hardness: 1246 MPa

Brinell Hardness: 1100 MPa

Most stable isotopes: Rh-103 (monoisotopic element)

Discoverer: William Wollaston, English (1803)

Rhodium, a chemical element with atomic number 45, belongs to the group of transition metals and is included in the select set of noble and precious metals that comprise the so-called platinum group. These metals, which include platinum itself, palladium, iridium, and osmium, share a remarkable corrosion resistance and chemical stability that allows them to be found in nature forming natural alloys among themselves, known as "native alloys". Such alloys usually appear as small nuggets in alluvial deposits or embedded in refractory minerals, and it is rare to find them accompanied by elements outside the family due to the extraordinary chemical inertness that characterizes these metals.

The discovery of rhodium occurred in 1803 thanks to the English chemist William Hyde Wollaston, the same person who identified palladium. Wollaston was working with a mineral sample from the "New World", probably South America, although the exact location has been lost in time. At that time, platinum was already known in Europe since Antonio de Ulloa described it in the 18th century, but it was suspected that the natural alloys containing it could harbor other unknown metals. Wollaston proceeded to dissolve the alloy in aqua regia, a mixture of acids capable of dissolving platinum and palladium, but not all metals of the group. Thus, after separating the dissolved platinum and palladium, an insoluble residue remained, containing osmium, iridium, ruthenium, and, of course, rhodium. It was when examining the resulting compounds of the latter that he observed they presented a very peculiar pink color, a circumstance that inspired him to name it "rhodium", from the Greek rhodon, "rose". This hue, along with its resistance to dissolution, was one of the key clues to differentiate it from other metals of the same group.

At some later point, certain publications confused the origin of the name with the British colony of Rhodesia, linking the discovery to Cecil Rhodes. However, this hypothesis is easily disproven: the colony received this name in 1888, eighty-five years after Wollaston isolated the metal. It is evident that the etymology of rhodium has nothing to do with colonial geography and everything to do with the unique chromatic singularity of its salts.

During its first decades of existence as an identified element, rhodium did not spark a scientific fever comparable to that of platinum or gold. It was more of a curious laboratory guest, a rarity without massive industrial applications. Its most immediate utility was found in the coating of metals more prone to tarnishing, as rhodium maintains its luster exceptionally even after long periods of exposure. Over time, and especially in the 20th century, this property would make it a valuable resource for jewelers and manufacturers of precision instruments.

Natural reserves of rhodium are concentrated mainly in Southern Africa, a region where complex mining operations extract platinum and other associated metals. On a smaller scale, it can be obtained as a byproduct of nickel ore processing, though in very reduced quantities. Obtaining it is complex and costly, which contributes to its high price in international markets.

Regarding its chemistry, rhodium does not form stable carbides or nitrides, unlike some marginal metals, but it can generate solid alloys with most transition metals, except those belonging to groups 3, 4, and 12, with which it shows little affinity. Its ability to combine in any proportion with refractory metals —with the exception of zirconium and hafnium—, with ferrous metals like iron, nickel, and cobalt, as well as with the other members of the platinum group, is well documented. Among metallurgical curiosities, the formation, together with silver, of a material known in Japan as "pseudo-palladium", whose properties astonishingly imitate those of real palladium, stands out. However, the compatibility of rhodium with group 11 metals —copper, silver, and gold— has been little explored, perhaps due to its scarcity and high cost. Nevertheless, some experimenters —myself included— have tried to investigate the possibility of a copper-rhodium alloy, motivated by the coincidence of their crystalline structure. Despite this, the tests carried out so far have yielded unpromising results or ones of little practical interest.

In terms of abundance, rhodium is one of the rarest metals in the Earth's crust: only iridium and rhenium can compete with it in scarcity, although certain estimates place it even below iridium in natural availability. Its rarity, combined with its technical and aesthetic qualities, has made it a metal of great commercial value. However, its price is characterized by extreme volatility: throughout the 20th century, rhodium did not surpass gold in quotation, but in recent years it has positioned itself as the most expensive metal in the world. This meteoric rise, however, can reverse with equal rapidity, which is why many consider its acquisition as an investment to entail significant risks. Therefore, and except for those seeking its use in industrial applications or high-end jewelry, it is usually recommended to focus investments on more stable metals such as gold, platinum, or silver. Thus, rhodium has gone from being an isolated curiosity to an occasional protagonist in the economic and technological scene, though always with that air of rarity reserved only for nature's most elusive metals.

Rhodium is a silvery-white metal, brilliant and noble, whose surface retains its luster with stubborn tenacity against the passage of time and weathering. At first glance, it could be confused with pure silver, but unlike silver, rhodium does not darken easily, not even in the presence of sulfur or sulfurous compounds that quickly blacken other metals. Mechanically, it possesses considerable hardness and a density slightly greater than that of lead. Its crystalline structure is face-centered cubic, as might be expected from its position in the periodic table, and although it is the most "workable" of its immediate group, it is still not an especially ductile or malleable metal: it is rather rigid and in certain contexts even fragile compared to palladium, platinum, silver, or gold, which debunks the popular belief that it behaves like an easily manipulated "jewelry metal".

This confusion is due to the fact that it is rarely used as a main component of alloys and much more often as a coating. Since the early 19th century, its ability to form thin, hard, and brilliant layers on other metals has been exploited, giving them a noble appearance and extraordinary corrosion resistance. The most visible example is so-called white gold, which is nothing more than an alloy of gold with a light-toned metal coated with a thin layer of rhodium that gives it its characteristic color. This coating not only maintains its shine but resists the action of acids and does not generate passive oxide layers as happens with titanium or tantalum; nor does it absorb hydrogen like palladium, nor does it react easily with elemental sulfur.

In terms of its abundance, rhodium is one of the rarest metals in the Earth's crust, much rarer than gold, and almost always appears in its native state or as part of natural alloys with other members of the platinum group. The minerals that contain it are extremely rare and, generally, little studied. Its tenacity is acceptable —allowing, for example, to work it by hammering—, but its lack of plasticity limits the manufacture of complex objects in a solid state. Forging a platinum-rhodium ring is possible, but extremely costly and technically demanding, which is why such pieces are more a symbol of prestige than a functionally superior product.

When used as an alloying agent, it is primarily in combination with palladium or platinum to harden them, although this function usually falls to ruthenium or iridium, which are more economical and behave similarly. In jewelry, the value of rhodium lies not so much in exceptional mechanical properties as in its rarity, its unalterable luster, and its indispensable role in the finish of white gold. Outside the jewelry sector, its most notable use is in the automotive industry, where it is part of three-way catalytic converters, but its high price and limited availability have kept its field of application restricted.

Rhodium stands out as one of the most corrosion-resistant metals, a characteristic that solidifies it as a noble material par excellence. At room temperature, it does not form oxides, a property that distinguishes it from most metals. Its resistance extends to almost all acids, whether reducing or oxidizing, although it reacts slowly with aqua regia, a corrosive mixture capable of attacking even precious metals. During its smelting, rhodium absorbs oxygen, a behavior similar to that of copper, but, unlike copper, it expels the oxygen upon solidification, releasing it in the form of generally non-toxic vapors. This peculiarity occurs only in its liquid state, highlighting its chemical stability in the solid state.

Unlike ruthenium, its predecessor in the periodic table with atomic number 44 versus rhodium's 45, this metal shows remarkable resistance to alkalis and bases, even at elevated temperatures. Furthermore, by not forming carbides, it can be melted in graphite crucibles without risk of chemical contamination. This combination of properties makes it an ideal material for high-demand applications, such as protective coatings in jewelry, optical mirrors, and industrial catalysts, where its durability and exceptional brilliance are highly valued.

Rhodium, one of the rarest metals in the Earth's crust, is distinguished by its scarcity, surpassed only by its exclusivity in specific applications. Although it is speculated that it might be more abundant in the deeper layers of the Earth, similar to other iron-affinity metals, its limited availability and high cost restrict its use to high-precision sectors. As a member of the platinum group metals, rhodium plays a crucial role as a catalyst in the automotive industry, where it reduces CO₂ emissions. However, its high price often leads manufacturers to opt for more economical alternatives, such as palladium.

The most common use of rhodium is in the coating of other metals, a process that highlights its exceptional brilliance, more durable and spectacular than that of silver or palladium. In jewelry, the characteristic color of white gold does not come from alloys with palladium, nickel, or other whitening agents, but from a rhodium plating that provides a brilliant and resistant finish. This attribute underscores two key aspects of the metal. First, rhodium surpasses other white metals in terms of aesthetics and durability, second only to platinum. Second, its scarcity and cost make it impractical as a primary alloying agent. Instead of directly alloying rhodium with gold, which could generate a superior alloy even with percentages of palladium, copper, nickel, or zinc, jewelers prefer to use it as a coating. With a small amount of rhodium, it is possible to plate several pieces of jewelry, achieving a visually similar finish to a solid alloy, but at a fraction of the cost. This practice, though efficient, reflects the balance between rhodium's exclusivity and the limitations imposed by its price and rarity.

Rhodium, whose name evokes a pinkish hue derived from the Greek "rhodon" (rose), shares a curious nominal relationship with certain minerals that do not contain this metal but reflect its etymology. One of them is rhodonite, a relatively abundant mineral sometimes classified as a semi-precious gemstone, although it is more appreciated in its raw state, without the polishing process known as "tumbling". This mineral, characterized by its attractive pink color, does not contain rhodium, but is an inosilicate with a complex chemical formula: (Mn, Fe, Mg, Ca)SiO₃. Its pinkish hue, reminiscent of rhodium's name, is purely coincidental, but reinforces the aesthetic connection between the two.

Similarly, there is another less known mineral, rhodolite, which also bears the mark of the pink color in its name. This nesosilicate, with the chemical formula (Mg, Fe)₃Al₂(SiO₄)₃, belongs to the garnet group and shares the same etymological root as rhodium and rhodonite. Although its composition does not include rhodium, its pinkish appearance links it nominally to the metal, creating an interesting parallelism in the world of mineralogy. These names, inspired by the pink color, illustrate how nature and science often converge in poetic terms, though not always in their chemical composition.