Ruthenium (Ru)
Ruthenium (Ru) is a transition metal with atomic number 44 and a standard atomic weight of 101.07. At room temperature it is a solid that melts at 2,333 °C and boils at 4,147 °C.
Ruthenium key facts
Ruthenium at a glance
Identity
| Symbol | Ru |
|---|---|
| Atomic number (Z) | 44 |
| Atomic weight | 101.07 ± 0.02 (CIAAW 2024) |
| Category | Transition metal |
| Group | 8 |
| Period | 5 |
| Block | d-block |
| CAS number | 7440-18-8 |
Atomic properties
| Electron configuration | [Kr] 4d7 5s1 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d7 5s1 |
|---|---|
| Electrons per shell | 2, 8, 18, 15, 1 |
| Oxidation states | +3, +4 Less common: -4, -2, +1, +2, +5, +6, +7, +8 |
| Electronegativity (Pauling) | 2.2 |
| First ionisation energy | 7.361 eV (710.2 kJ/mol) 2nd: 16.76 eV · 3rd: 28.47 eV |
| Electron affinity | 1.05 eV |
| Atomic radius (empirical) | 130 pm |
| Covalent radius | 125 pm |
| Van der Waals radius | 213 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 2,333 °C · 4,231.4 °F · 2,606.15 K |
| Boiling point | 4,147 °C · 7,496.6 °F · 4,420.15 K |
| Density | 12.1 g/cm³ |
| Specific heat capacity | 0.238 J/(g·K) |
| Thermal conductivity | 117 W/(m·K) |
| Crystal structure | Hexagonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 102Ru 31.55%, 104Ru 18.62%, 101Ru 17.06%, 99Ru 12.76%, 100Ru 12.6%, 96Ru 5.54% |
| Stable isotopes | 7 |
| Known isotopes | 41 |
| Abundance in Earth's crust | 0.001 mg/kg |
| Abundance in seawater | 0.0000007 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Ruthenium electron configuration
Ruthenium has 44 electrons. Written in short form, its ground-state configuration is [Kr] 4d7 5s1 — the electrons of the noble gas krypton, plus 4d7 5s1.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d7 5s1.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 5s2, 4d6, but ruthenium actually has 5s1, 4d7. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1844 — Karl Ernst Claus · Russia.
Latin Ruthenia, Russia.
Known for
A hard platinum-group metal used in electrical contacts, hard-disk layers and catalysts; hardens platinum and palladium alloys.
Isotopes of ruthenium
Naturally occurring ruthenium is a mix of 7 isotopes. Each has 44 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 102Ru | 44 | 58 | 31.55% | stable |
| 104Ru | 44 | 60 | 18.62% | stable |
| 101Ru | 44 | 57 | 17.06% | stable |
| 99Ru | 44 | 55 | 12.76% | stable |
| 100Ru | 44 | 56 | 12.6% | stable |
| 96Ru | 44 | 52 | 5.54% | stable |
| 98Ru | 44 | 54 | 1.87% | stable |
41 isotopes of ruthenium are known in our data set (half-lives as published in nuclear data tables).
Ruthenium compared with group 8
The other elements in group 8, from top to bottom of the table: