Rhodium: Properties, Isotopes, and Key Applications

Together with elements like iridium, osmium, ruthenium, palladium and platinum, rhodium is an important member of a family of special elements, otherwise known as Platinum Group Metals (PGM). This is a vital topic in the syllabus and consequently, it is imperative that you understand this element.

Furthermore, you should also be wary of the varying physical and chemical properties of this element too. Discussed below in detail, also attempt the different questions which are provided for your practise.

What is Rhodium?

The element Rhodium, pronounced as Ro-dee-um, is signified by its chemical symbol Rh and has an atomic number 45. Rhodium has a silvery white appearance in its natural form and is resistant to corrosion. Being one of the noble metals of the PGM group, rhodium is chemically inert and is one of the rarest and the most precious metals. In nature, it occurs mostly as a free metal or in other metal ores. 

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What Does Rhodium Mean?

Now that we are familiar with some of the basic information regarding what is rhodium, let us jump on to the origin and history of Rhodium. 

The word Rhodium is derived from the Greek word ‘rhodon’, which means ‘rose-coloured’. Rhodium was first discovered by British scientist William Hyde Wollaston and chemist Smithson Tennant in 1803, both of whom were commercially digging ores for selling. 

Wollaston found rhodium in a platinum ore, which he procured from South America and which contained platinum, palladium and rhodium. He subjected the ore to complex chemical processes using various other compounds like sodium hydroxide, ammonium chloride and zinc to finally filter out rhodium as a free metal. The element got its name due to its rose-coloured appearance, when it was first filtered out. 

Though rhodium was not a very popular element at the time of its discovery, in about 100 years, scientists started using the metal to involve it in various experiments which required high temperatures. The most important usage of rhodium today started in 1970, when it is was first used inside catalytic converters to reduce the pollutant level in exhaust emissions. 

Pop Quiz 1

1. What is Rhodium?

1. A noble gas

2. A noble metal (Answer)

3. A highly corrosive metal

4. A compound used in nuclear reactions

Physical Properties of Rhodium 

Some of the most important physical properties of rhodium are listed below. 

1. Rhodium is silvery-white in colour and is extremely hard and durable. 

2. Rhodium has a metallic lustre and high reflectance. 

3. The metal has an atomic mass of 102.906 AMU. 

4. It has a melting point of 1963 degrees Celcius and a boiling point of 3695 degrees Celcius. 

5. It remains solid under room temperatures and has a density of 12.4 g/cm3. 

6. Though rhodium does not generally form an oxide even after heating, it absorbs oxygen only when it reaches its melting point. The absorbed oxygen is then released when it is solidified back. 

7. Rhodium is mostly insoluble in all acids like nitric acid. It is only slightly soluble in aqua regia, a mixture of nitric and hydrochloric acid. 

Chemical Properties of Rhodium

With a brief idea on the element’s physical properties and about what type of metal is rhodium, let us move on to the most important section for any element – its chemical properties. Below we make a list of some of the most important chemical properties of rhodium. 

1. The number of electrons per shell of the element is 2, 8, 18, 16, 1. Hence, even if rhodium belongs to group 9 of the periodic table, this particular configuration is anomalous for the group. The same aberration is also found in its neighbouring elements including palladium (46), niobium (41) and ruthenium (44). 

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2. The most common oxidation state of the compound is +3, or the element loses three of its electrons to become an ion. This oxidation state varies from 0 to +6.

3. Due to its inert nature, rhodium, like some of its sister elements, do not form any volatile oxides and most of its oxides are stable. Some of its popular oxides are RhO2, Rh2O3 and RhO2.xH2O. 

4. Rhodium forms many halogen compounds though. Some of its most popular halogen compounds include rhodium(V) fluoride, rhodium(VI) fluroide and rhodium(III) chloride, all of which use the higher level oxidation states of the element. The compounds formed in the lower level oxidation states are stable only with ligands. 

5. Wilkinson’s Catalyst is the most widely used halogen compound of rhodium. The chemical formula for Wilkinson’s Catalyst is C54H45ClP3Rh and is known in words as chloridotris(triphenylphosphine)rhodium(I). This compound is mainly used for hydrogenation of alkenes. 

Isotopes of Rhodium

Rhodium, as a free metal, occurs in only one naturally occurring isotope, 103Rh. Despite this, rhodium can occur as some stable radioisotopes with longer half-lives and other twenty odd of them with half-lives less than an hour. The most stable radioisotopes of the element are 99Rh (half life – 16 days), 102mRh (half life – 2.9 years), 102Rh (half life – 207 days) and 101Rh (half life – 3.3 years). 

In the isotopes of the element which have atomic weight less than 103, rhodium decays via electron capture and the final decay product is ruthenium. While in isotopes having weight more than 103, it decays via emitting beta particles and the final decay product is palladium. 

Applications and Uses of Rhodium

With all of its properties and other trivia about the element discussed above, the only other question that remains is what is rhodium used for. 

1. As a Catalytic Converter

A catalytic converter is a compound which is used inside vehicles to convert the unburned carbon monoxide, nitrogen oxide and other hydrocarbons into less toxic gases, such that the emissions can then be sent out into the atmosphere, without any environmental issue. With global warming and climate change being two of the most burning topics debated by scientists and activists worldwide, catalytic converters are absolutely essential nowadays for vehicle manufacturers. Surveys suggest that almost 80 per cent of rhodium used in 2012 has been for manufacturing catalytic converters. 

The generic chemical reaction which rhodium catalyses inside vehicles is 2 NOx → x O2 + N2, in which rhodium facilitates the reduction of harmful nitrogen oxides into gases like oxygen and nitrogen which can be easily emitted into the atmosphere. 

2. In Jewellery and Ornaments Industry

Rhodium also finds itself in decoration and jewellery making. Jewellery flashing is a popular usage of the element in the industry, where the metal is electroplated on gold and platinum to render ornaments a shiny look and feel. The shine usually wears off when used for a certain period of time. It is also used to coat sterling silver to save it from tarnishing. Due to its rarity, price and high melting point, rhodium is mostly used as a coating agent rather than being used as solid metal. 

Did You Know?

Rhodium is also used to signify honour in many prizes and awards. The Guinness Book of World Records awarded a rhodium-plated vinyl to the famous singer and songwriter Paul McCartney, one of the founding members of The Beatles, in 1979 for being the highest selling artist of all time. 

3. Other Miscellaneous Uses

Some of other rare uses of rhodium are listed below. 

1. It is used for hardening and improving corrosion threshold of metals like platinum and palladium. This manufactured alloy of platinum and rhodium or palladium and rhodium are then used mostly in glass industries, making electrodes for aircraft and laboratory crucibles. 

2. Due to its low electrical resistance, the element is also used for making electrical contacts. 

3. It is used in producing characteristic X-rays in mammography systems. 

4. Rhodium finds high amount of usage in nuclear reactors to measure neutron flux levels. 

5. Vehicle manufacturers use the element in constructing headlight reflectors. 

Pop Quiz 2

1. Majorly, What is Rhodium Used for?

1. Making catalytic converters (Answer)

2. Making glass bodies

3. Making electrical cables

4. As a coating agent for iron and zinc

For Your Convenience, here is Table Containing all the Necessary Information Regarding Rhodium. 

So, this was all about what is rhodium and all its chemical and physical properties along with its applications and uses. To know more about the other chemical compounds and any other topic of chemistry, download the app or visit the Vedantu website today to get comprehensive guides and informative PDFs regarding the same.