Chemical element · reference page

45Rh

Rhodium

Rhodium is element 45 (Rh), located in period 5, group 9 and the d block. At room temperature its reference phase is solid. Its electron configuration is [Kr] 4d8 5s1.

Atomic number45

Standard atomic weight102.90549(2)

Group9

Period5

Blockd

Electron configuration[Kr] 4d8 5s1

Electronic structure

What controls its chemistry

The d subshell participates in the chemistry of rhodium. Transition metals can use electrons of similar energies in bonding, which is why variable oxidation states, coordination compounds and catalytic behaviour are common across the d block.

Its electron configuration, [Kr] 4d8 5s1, should be read together with the oxidation state of the compound: removing electrons changes which d orbitals are occupied and can alter colour, magnetism, bonding and reactivity.

Chemical forms

Compounds, ions and materials

For rhodium, important chemical forms include metals and alloys, oxides, halides and coordination compounds. Variable oxidation states are common because s and d electrons have similar energies, and ligands around the metal ion can strongly alter colour, magnetism and reactivity.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 12.41 g/cm³. The melting point is 2 236 K (1 963 °C). The boiling point is 3 968 K (3 695 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where rhodium is found and used

In practice, rhodium is encountered in catalysts, reflectors and jewellery.

In practice

Examples of real uses

Catalysts

Rhodium is used to accelerate chemical reactions without being consumed in the overall reaction.

Reflectors

Rhodium-containing reflective, corrosion-resistant surfaces or alloys are used in specialised optical and high-temperature reflectors.

Jewellery

Rhodium is used for colour, corrosion resistance, rarity, polish or mechanical properties in jewellery alloys.

Periodic position

What its neighbours tell us

Rhodium is in period 5 and group 9. In the neighbourhood shown here, it lies between ruthenium and palladium in the same period; cobalt is above it and iridium below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationtransition metal

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 16 · 1

Density12.41 g/cm³

Melting point2 236 K (1 963 °C)

Boiling point3 968 K (3 695 °C)

Pauling electronegativity2.28

First ionisation energy719.7 kJ/mol

Electron affinity110.3 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of rhodium is associated with William Hyde Wollaston. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 102.90549(2). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Rhodium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with rhodium depend on chemical form, dose and route of exposure. Pure rhodium, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.