GROUP 9 TRANSITION METAL

Rhodium is the second of the six metals that make up the platinum group (see “Ruthenium, Ru” on page 129 for the full list). Also known as noble metals, these six metals display a somewhat regal disdain for reacting with such mere commoners as air and water. This aloof inertness is even more pronounced in the case of rhodium, which resists corrosion from all acids up to 100°C, including aqua regia.⁵

Rhodium is a hard, dense, and lustrous metal that takes its name from the Greek word rhodon, meaning rose colored. Its name derives from the rose-red crystals of its chloride salt (RhCl₃), which William Hyde Wollaston (1766–1828) generated en route to discovering the metal in 1803. Rhodium’s rarity (it’s only the 79th most abundant element in Earth’s crust) is in keeping with the other noble metals. It is particularly prized for the exceptional reflectivity of its micrometer-thin surface films, used to coat optical mirrors, headlight reflectors, and shiny rhodium-plated jewelry.

The six platinum group metals congregate in an orderly 3 × 2 matrix nestled close to the middle⁶ of the periodic table. Rhodium sits proudly in the center of the top row of this matrix.

Rhodium is only found as a minor constituent in other metals’ ores. For example, its sparing presence in platinum ores dictates that unless the market demand for platinum is strong, it’s simply not worth refining the ore to extract the rhodium. Occasionally, this has led to wild fluctuations in the price of rhodium, prompting small fortunes to be made or lost depending on the timing of the purchase.

Rhodium has a well-regarded environmental pedigree, because it helps control the noxious emissions from car exhausts. In fact, more than 80 percent of rhodium ends up in the three-way catalytic convertors in cars. There it chemically converts undesirable nitrogen oxides (also known as NO_x, meaning NO and NO₂) to nitrogen (N₂) and oxygen (O₂).