Rhodium

Rhodium (Rh) is a transition metal with atomic number 45 and an atomic mass of 102.91. Its electron configuration is [Kr] 4d8 5s1.

Rhodium key data

Symbol	Rh
Atomic number	45
Atomic mass (standard atomic weight)	102.91
Molar mass	102.91 g/mol
Protons	45
Neutrons (most abundant isotope, rhodium-103)	58
Electrons (neutral atom)	45
Category	Transition metal
Period	5
Group	9
Block	d-block
Noble gas notation	[Kr] 4d8 5s1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d8 5s1
Electrons per shell	2, 8, 18, 16, 1

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of rhodium

From PubChem, the US National Institutes of Health. Public domain.

Melting point	2237 K (1963.9 °C), 13th of 103 by melting point
Boiling point	3968 K (3694.9 °C), 18th of 93 by boiling point
Density	12.4 g/cm³, 17th of 96 by density
State at room temperature	Solid
Electronegativity	2.28 (Pauling), joint 15th of 95 by electronegativity
Van der Waals radius	195 pm, 66th of 99 by atomic radius
First ionisation energy	7.459 eV (719.7 kJ/mol), 44th of 102 by first ionisation energy
Electron affinity	1.137 eV (109.7 kJ/mol), 20th of 57 by electron affinity
Common oxidation states	+3
Discovered	1803

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Rhodium electron configuration

The full electron configuration of rhodium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d8 5s1. With the noble-gas core collapsed it is [Kr] 4d8 5s1, and the outermost subshell is 5s1.

Rhodium does not follow the simple filling order. Its measured configuration differs from the one the aufbau rule predicts, because its 4d and 5s subshells lie so close in energy that the simple filling order cannot predict which arrangement is lowest. This is a measured fact about the atom rather than an exception to be derived.

Bohr model of rhodium (Rh): a nucleus and five shells holding 2, 8, 18, 16 and 1 electrons from the inside out. — Bohr model of rhodium (Rh). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of rhodium (Rh), [Kr] 4d8 5s1, with the krypton core drawn as one bar: one box per orbital, filled by Hund’s rule, showing three unpaired electrons, in 4d and 5s. — Orbital diagram of rhodium (Rh). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one rhodium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 102.91 / (6.02214076 × 10²³)
= 1.7089 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 102.91
= 5.8519 × 10²¹ atoms

Rhodium on the periodic table

Rhodium is in period 5, group 9, in the d-block.

The other elements of group 9 are cobalt, iridium and meitnerium.