Transition metalElement 45 · d-block

Rhodium (Rh)

Rhodium (Rh) is a transition metal with atomic number 45 and a standard atomic weight of 102.91. At room temperature it is a solid that melts at 1,963 °C and boils at 3,695 °C.

45RhRhodium102.91

Atomic number

protons

Atomic weight

102.91

CIAAW 2024

Melting point

1,963 °C

2,236.15 K

Boiling point

3,695 °C

3,968.15 K

Density

12.4 g/cm³

near room temperature

Electronegativity

2.28

Pauling scale

State at 25 °C

Solid

Discovered

1803

England

Rhodium at a glance

Identity

Symbol	Rh
Atomic number (Z)	45
Atomic weight	102.91 ± 0.01 (CIAAW 2024)
Category	Transition metal
Group	9
Period	5
Block	d-block
CAS number	7440-16-6

Atomic properties

Electron configuration	[Kr] 4d⁸ 5s¹ Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁸ 5s¹
Electrons per shell	2, 8, 18, 16, 1
Oxidation states	+3 Less common: -3, -1, +1, +2, +4, +5, +6, +7
Electronegativity (Pauling)	2.28
First ionisation energy	7.459 eV (719.7 kJ/mol) 2nd: 18.08 eV · 3rd: 31.06 eV
Electron affinity	1.137 eV
Atomic radius (empirical)	135 pm
Covalent radius	125 pm
Van der Waals radius	210 pm

Physical properties

State at 25 °C	Solid
Melting point	1,963 °C · 3,565.4 °F · 2,236.15 K
Boiling point	3,695 °C · 6,683 °F · 3,968.15 K
Density	12.4 g/cm³
Specific heat capacity	0.243 J/(g·K)
Thermal conductivity	150 W/(m·K)
Crystal structure	Face-centred cubic

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	¹⁰³Rh 100%
Stable isotopes	2
Known isotopes	41
Abundance in Earth's crust	0.001 mg/kg

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Rhodium electron configuration

Rhodium has 45 electrons. Written in short form, its ground-state configuration is [Kr] 4d⁸ 5s¹ — the electrons of the noble gas krypton, plus 4d⁸ 5s¹.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁸ 5s¹.

An exception to the filling rule. The simple aufbau (Madelung) order predicts 5s², 4d⁷, but rhodium actually has 5s¹, 4d⁸. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.

4d↑↓↑↓↑↓↑↑

5s↑

Discovery and name

Discovered

1803 — William Hyde Wollaston · England.

Name

Greek rhodon, rose — for the colour of its salts.

Known for

One of the rarest and most expensive metals. Its main use is in catalytic converters to reduce nitrogen-oxide emissions.

Isotopes of rhodium

Naturally occurring rhodium is a mix of 1 isotope. Each has 45 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
¹⁰³Rh	45	58	100%	stable

41 isotopes of rhodium are known in our data set (half-lives as published in nuclear data tables).

Rhodium compared with group 9

The other elements in group 9, from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Cobalt	27	58.933	1,495 °C	2,927 °C	8.86 g/cm³	1.88
Rhodium	45	102.91	1,963 °C	3,695 °C	12.4 g/cm³	2.28
Iridium	77	192.22	2,446 °C	4,428 °C	22.562 g/cm³	2.2
Meitnerium	109	[278]	—	—	37.4 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of rhodium?

The standard atomic weight of rhodium is 102.91 ± 0.01 (CIAAW 2024), so one mole weighs about 102.91 grams.

How many protons, neutrons and electrons does rhodium have?

A neutral rhodium atom has 45 protons and 45 electrons. Its most common isotope, rhodium-103, has 58 neutrons.

What is the electron configuration of rhodium?

[Kr] 4d⁸ 5s¹ (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁸ 5s¹).

What are the melting and boiling points of rhodium?

Rhodium melts at 1,963 °C (3,565.4 °F) and boils at 3,695 °C (6,683 °F).

Is rhodium a metal?

Rhodium is a metal. It is in group 9, period 5, and is a solid at room temperature.

Who discovered rhodium?

1803 — William Hyde Wollaston (England).