Ruthenium

Ruthenium (Ru) is a transition metal with atomic number 44 and an atomic mass of 101.07. Its electron configuration is [Kr] 4d7 5s1.

Ruthenium key data

Symbol	Ru
Atomic number	44
Atomic mass (standard atomic weight)	101.07
Molar mass	101.07 g/mol
Protons	44
Neutrons (most abundant isotope, ruthenium-102)	58
Electrons (neutral atom)	44
Category	Transition metal
Period	5
Group	8
Block	d-block
Noble gas notation	[Kr] 4d7 5s1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d7 5s1
Electrons per shell	2, 8, 18, 15, 1

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

Measured properties of ruthenium

From PubChem, the US National Institutes of Health. Public domain. The discovery year is the year ruthenium was first isolated, from Wikipedia’s timeline of chemical element discoveries, in place of PubChem’s 1827.

Melting point	2607 K (2333.9 °C), 9th of 103 by melting point
Boiling point	4423 K (4149.9 °C), 12th of 93 by boiling point
Density	12.1 g/cm³, 18th of 96 by density
State at room temperature	Solid
Electronegativity	2.2 (Pauling), joint 17th of 95 by electronegativity
Van der Waals radius	207 pm, joint 52nd of 99 by atomic radius
First ionisation energy	7.361 eV (710.2 kJ/mol), 47th of 102 by first ionisation energy
Electron affinity	1.05 eV (101 kJ/mol), 24th of 57 by electron affinity
Common oxidation states	+3
Discovered	1844

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.

Ruthenium electron configuration

The full electron configuration of ruthenium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d7 5s1. With the noble-gas core collapsed it is [Kr] 4d7 5s1, and the outermost subshell is 5s1.

Ruthenium 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 ruthenium (Ru): a nucleus and five shells holding 2, 8, 18, 15 and 1 electrons from the inside out. — Bohr model of ruthenium (Ru). 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 ruthenium (Ru), [Kr] 4d7 5s1, with the krypton core drawn as one bar: one box per orbital, filled by Hund’s rule, showing four unpaired electrons, in 4d and 5s. — Orbital diagram of ruthenium (Ru). 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 ruthenium 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 = 101.07 / (6.02214076 × 10²³)
= 1.6783 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 101.07
= 5.9584 × 10²¹ atoms

Ruthenium on the periodic table

Ruthenium is in period 5, group 8, in the d-block.

The other elements of group 8 are iron, osmium and hassium.