Rhodium (Rh)

Rhodium is a hard, silvery-white, corrosion-resistant transition metal used in high-temperature alloys, catalysts (especially for automotive three-way catalytic converters), and reflective coatings. It is one of the rarest and most valuable precious metals.

Atomic Number
45
Atomic Mass
102.906
Phase (STP)
Solid
Block
D
Electronegativity (Pauling)
2.28

Bohr Atomic Model

Protons
45
Neutrons
58
Electrons
45
Identity
Atomic Number45
SymbolRh
NameRhodium
Group9
Period5
Position
Period5
Group Label9
Grid X9
Grid Y5
Physical Properties
Atomic Mass (u)102.906
Density (g/cm³)12.4
Melting Point (K)2236 K 1964 °C
Boiling Point3968 K 3695 °C
Phase at STPSolid
CategoryTransition Metals
Liquid Density (g/cm³)—
Molar Volume (cm³/mol)8.3
Emission Spectrum (nm)—
Discovery
English NameRhodium
English Pronunciationˈroʊdiəm
Latin NameRhodium
Latin PronunciationRO-dee-um
Year1803
DiscovererWilliam Hyde Wollaston
CountryEngland
CAS Number7440-16-6
CID Number23948
RTECS Number—
Atomic Properties
Electron ShellK2 L8 M18 N16 O1
Electron Configuration[Kr] 4d^85s^1
Oxidation States-1 +1 +2 +3 +4 +5 +6
Ion ChargeRh+, Rh3+
Ionization Potential (eV)7.459
Electronegativity (Pauling)2.28
Electron Affinity (kJ/mol)109.704
Electrons45
Protons45
Neutrons58
ValenceI–III
BlockD
Atomic Radius (pm)134
Covalent Radius (pm)134
van der Waals Radius (pm)210
Thermodynamic Properties
PhaseSOLID
Heat of Fusion (kJ/mol)26.6
Specific Heat (J/g·K)0.243
Thermal Expansion (1/K)0
Heat of Vaporization (kJ/mol)493
Mechanical Properties
Brinell Hardness—
Mohs Hardness6
Vickers Hardness1246
Bulk Modulus (GPa)275
Young's Modulus (GPa)380
Shear Modulus (GPa)150
Poisson Ratio0.26
Sound Speed (m/s)—
Refractive Index—
Thermal Conductivity (W/m·K)150
Electromagnetic Properties
Electrical Conductivity (S/m)23000000
Electrical TypeCONDUCTOR
Magnetic TypePARAMAGNETIC
Volume Magnetic Susceptibility—
Mass Magnetic Susceptibility—
Molar Magnetic Susceptibility—
Resistivity (Ω·m)0
Superconducting Point (K)—
Crystal Properties
StructureFace-centered cubic (fcc)
SystemCUBIC
Space GroupFm-3m (No. 225)
a (Å)3.803
b (Å)3.803
c (Å)3.803
α (°)90
β (°)90
γ (°)90
Debye Temperature (K)480
Nuclear Properties
RadioactiveNo
Half-life—
Lifetime—
Neutron Cross-section (barn)—
Safety Information
Health Hazard—
Reactivity Hazard—
Specific HazardFinely divided dust may be flammable; many rhodium salts are irritants.
Prevalence
Universe—
Sun—
Oceans—
Human Body—
Earth Crust0
Meteorites—


FAQs about Rhodium

Rhodium exhibits several oxidation states, most commonly 0, +1, and +3. Representative compounds include:

  • Rh(0): carbonyls like \(\mathrm{Rh_6(CO)_{16}}\).
  • Rh(I): Wilkinson's catalyst \(\mathrm{RhCl(PPh_3)_3}\), d8 square-planar.
  • Rh(III): \(\mathrm{RhCl_3}\), \(\mathrm{[Rh(H_2O)_6]^{3+}}\), typically octahedral.

Rhodium is exceptionally effective for NOx reduction while Pt/Pd handle CO and hydrocarbons. A simplified reaction pathway is:

\(\mathrm{2\,NO + 2\,CO \xrightarrow[\text{Rh}]{ } N_2 + 2\,CO_2}\)

Its activity and sulfur tolerance help meet stringent emissions standards.

The accepted ground-state configuration is \([\mathrm{Kr}]\,4d^8\,5s^1\), which is an "anomalous" arrangement versus the naive Aufbau expectation \(4d^7\,5s^2\). Exchange stabilization favors one electron in 5s and eight in 4d.

Rhodium is typically a by-product of platinum-group metal (PGM) and Ni/Cu sulfide ore refining. After primary metal extraction, the PGM residue is chemically separated (chlorination/solvent extraction/precipitation) to isolate Rh, which is then reduced to metal or formulated as salts (e.g., \(\mathrm{RhCl_3}\)).

Rhodium has high reflectivity in the visible range, exceptional corrosion resistance, and maintains surface integrity at elevated temperatures. Rh plating on mirrors, optical components, and electrical contacts provides wear resistance and low tarnish.

Important systems include:

  • Wilkinson's catalyst \(\mathrm{RhCl(PPh_3)_3}\) for homogeneous hydrogenation of alkenes.
  • Rh–hydride carbonyls (e.g., \(\mathrm{HRh(CO)(PPh_3)_3}\)) for hydroformylation.
  • Various Rh(III) C–H activation catalysts for selective functionalization.

\(\mathrm{R{-}CH{=}CH_2 + CO + H_2 \xrightarrow[\text{Rh}]{ } R{-}CH_2{-}CH_2{-}CHO}\)

All three are PGMs with overlapping roles, but Rh is particularly strong for NOx reduction and certain asymmetric hydrogenations. Pd dominates cross-couplings (Suzuki/Heck/Sonogashira), while Pt is widely used for hydrosilylation and reforming. Choice depends on substrate scope, selectivity, and operating conditions.

Bulk metallic Rh is considered low in toxicity and relatively inert. However, finely divided Rh and some soluble Rh salts/complexes can be harmful if inhaled or ingested. Standard lab/industrial hygiene (gloves, fume hood, dust control) and waste management are required.

Several factors drive pricing: extreme scarcity, concentration of supply in a few regions, complex refining logistics, and demand spikes tied to emissions regulations (automotive catalysts). Small supply disruptions can cause large price swings.

Rhodium crystallizes in a face-centered cubic (fcc) lattice with high melting point (\(\approx\) 1964 °C) and excellent hardness. It is paramagnetic and shows high electrical and thermal conductivity, contributing to its performance in contacts and high-temperature alloys.