Rhodium (Rh)

• Element Symbol: Rh
• Atomic Number: 45
• Atomic Mass: 102.9055
• Group # in Periodic Table: 9
• Group Name: Transition metals
• Period in Periodic Table: 5
• Block of Periodic Table: d-block
• Discovered by: William Hyde Wollaston (1803)

Rhodium is a chemical element in the periodic table. It is considered a platinum-group metal, a classification that also includes ruthenium, palladium, osmium, iridium, and platinum. These platinum-group metals are all transition metals, which typically form colored compounds and have strong valence (the ability of an atom to connect chemically to another atom).

In the early 1800s, the French chemist Hippolyte-Victor Collet-Descotils discovered the platinum-group metal iridium independently of its primary discoverer, Smithson Tennant. After further observation of platinum-group metals and the salt compounds they formed, Collet-Descotils hypothesized that the red hue of these salts could be attributed to the presence of another element. Although he did not identify this unknown element himself, his theory inspired William Hyde Wollaston, an English chemist, to investigate further. In 1803 Wollaston obtained a sample of crude platinum from South Africa and dissolved it in aqua regia (nitro-hydrochloric acid). Dissolving the sample even more, Wollaston added the yields of the aqua regia solution to ammonium chloride, which itself yielded a liquid with intriguing properties. Continuing his experiments on this liquid, Wollaston eventually produced a dark red powder, which he believed to be a new element. He named this new element "rhodium" after the Greek word rhodon (rose), a nod to the color of rhodium salts.

Physical Properties

Rhodium is a silvery-white metallic color and is quite reflective. At 298 kelvins (K), rhodium’s standard state is a hard, resilient solid that rates a 6 on the Mohs hardness scale. In this solid state it has a density of 12.45 grams per cubic centimeter (g/cm³). Rhodium has extremely high resistance to corrosion and does not tarnish when exposed to air. The melting point of rhodium is 1964 degrees Celsius (°C), and its boiling point is 3695 °C. These values are both quite high, which is a common characteristic of many transition metals. The specific heat of rhodium is 240 joules per kilogram-kelvin (J/kg·K). Rhodium is a good conductor, with an electrical conductivity of 2.3 × 10⁷ siemens per meter (S/m). Its thermal conductivity is 150 watts per meter-kelvin (W/m·K). Rhodium is paramagnetic, which means that it is magnetized when placed within a magnetic field, but it does not retain this property upon removal.

Chemical Properties

The most common oxidation states of rhodium are +6, +3, and 0. Rhodium has a face-centered cubic crystal structure, and it is resistant to most acids. Naturally occurring rhodium is made up of a single isotope, rhodium-103, which is the element’s only stable isotope. Twenty-three radioactive isotopes have also been identified. The most stable of these isotopes are rhodium-101, rhodium-102, and rhodium-99, with half-lives of 3.3 years, 207 days, and 16.1 days, respectively. The remaining twenty radioactive isotopes generally have half-lives that are shorter than one hour, although there are a few exceptions.

Unstable rhodium isotopes with mass numbers less than 103 undergo electron capture, a form of radioactive decay in which one of the atom’s electrons is absorbed by the nucleus, resulting in the transformation of a proton into a neutron and a neutrino. In rhodium, this process usually results in ruthenium isotopes. Unstable rhodium isotopes with mass numbers greater than 103 undergo beta decay, another form of radioactive decay, in which a proton is converted into a neutron (or vice versa) inside the nucleus of an atom. When a rhodium atom goes through this process, energy is released in the form of a beta particle (a fast-moving electron or positron), resulting in isotopes of palladium.

Applications

Only about 0.0002 parts per million of Earth’s crust is made up of rhodium, making it one of the scarcest elements. Rhodium, which is often present alongside fellow platinum-group metals in other natural mineral sources, is usually found in the Ural Mountains of Russia or in parts of South Africa. In addition, the small amount—approximately 0.1 percent—of rhodium found in copper and nickel can be obtained as a by-product when refining the ores of these two elements. It is from these sources that rhodium is commercially mined; about sixteen metric tons are mined annually, and three thousand metric tons are held in reserves. Rhodium is generally considered to be nontoxic, although compounds of rhodium in particular are treated as highly toxic. Exposure to rhodium dust should be limited, and the inhalation of too much can be dangerous because the element acts as a carcinogen that increases the risk of developing cancer in those exposed. Rhodium compounds that come in contact with the skin leave a resilient stain.

Due to rhodium’s high strength and resistance to corrosion, the element is commonly used in alloys, particularly with palladium and platinum. These alloys are used to create spark plugs on airplanes as well as in furnace windings. Additionally, due to the element’s good electrical conductivity, rhodium’s most common practical application is in electrical contact materials, particularly in the catalytic converters found in motor vehicles. Rhodium plating is extremely durable and is used in microscopes as well as in the jewelry industry. White gold and sterling silver are often plated with rhodium for improved appearance and to help prevent tarnishing and corrosion.

Bibliography

Crabtree, Robert H. The Organometallic Chemistry of the Transition Metals. 6th ed. Hoboken: Wiley, 2014. Print.

Emsley, John. Nature’s Building Blocks: An A–Z Guide to the Elements. 2nd ed. New York: Oxford UP, 2011. Print.

Halka, Monica, and Brian Nordstrom. Transition Metals. New York: Facts on File, 2011. Print.

Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.

"Technical Data for Rhodium." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 11 Sept. 2015.