Ruthenium, 44Ru
Ruthenium a half bar.jpg
Pronunciationm/ (THEE-nee-əm)
Appearancesilvery white metallic
Standard atomic weight Ar, std(Ru)101.07(2)[1]
Ruthenium in the periodic table
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon


Atomic number (Z)44
Groupgroup 8
Periodperiod 5
Element category  Transition metal
Electron configuration[Kr] 4d7 5s1
Electrons per shell2, 8, 18, 15, 1
Physical properties
Phase at STPsolid
Melting point2607 K ​(2334 °C, ​4233 °F)
Boiling point4423 K ​(4150 °C, ​7502 °F)
Density (near r.t.)12.45 g/cm3
when liquid (at m.p.)10.65 g/cm3
Heat of fusion38.59 kJ/mol
Heat of vaporization619 kJ/mol
Molar heat capacity24.06 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)258828113087342438454388
Atomic properties
Oxidation states−4, −2, 0, +1,[2] +2, +3, +4, +5, +6, +7, +8 (a mildly acidic oxide)
ElectronegativityPauling scale: 2.2
Ionization energies
  • 1st: 710.2 kJ/mol
  • 2nd: 1620 kJ/mol
  • 3rd: 2747 kJ/mol
Atomic radiusempirical: 134 pm
Covalent radius146±7 pm
Color lines in a spectral range
Spectral lines of ruthenium
Other properties
Natural occurrenceprimordial
Crystal structurehexagonal close-packed (hcp)
Hexagonal close packed crystal structure for ruthenium
Speed of sound thin rod5970 m/s (at 20 °C)
Thermal expansion6.4 µm/(m·K) (at 25 °C)
Thermal conductivity117 W/(m·K)
Electrical resistivity71 nΩ·m (at 0 °C)
Magnetic orderingparamagnetic[3]
Magnetic susceptibility+39·10−6 cm3/mol (298 K)[3]
Young's modulus447 GPa
Shear modulus173 GPa
Bulk modulus220 GPa
Poisson ratio0.30
Mohs hardness6.5
Brinell hardness2160 MPa
CAS Number7440-18-8
Namingafter Ruthenia (Latin for: medieval Kyivska Rus' region)
Discovery and first isolationKarl Ernst Claus (1844)
Main isotopes of ruthenium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
97Rusyn2.9 dε97Tc
103Rusyn39.26 dβ103Rh
106Rusyn373.59 dβ106Rh
| references

Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemicals. Russian-born scientist of Baltic-German ancestry Karl Ernst Claus discovered the element in 1844 at Kazan State University and named ruthenium in honor of Russia (Ruthenia is the Latin name of Rus). Ruthenium is usually found as a minor component of platinum ores; the annual production has risen from about 19 tonnes in 2009 [4] to some 35.5 tonnes in 2017.[5] Most ruthenium produced is used in wear-resistant electrical contacts and thick-film resistors. A minor application for ruthenium is in platinum alloys and as a chemistry catalyst. A new application of ruthenium is as the capping layer for extreme ultraviolet photomasks. Ruthenium is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario and in pyroxenite deposits in South Africa.[6]


Physical properties

Gas phase grown crystals of ruthenium metal.

Ruthenium, a polyvalent hard white metal, is a member of the platinum group and is in group 8 of the periodic table:

Z Element No. of electrons/shell
26 iron 2, 8, 14, 2
44 ruthenium 2, 8, 18, 15, 1
76 osmium 2, 8, 18, 32, 14, 2
108 hassium 2, 8, 18, 32, 32, 14, 2

Whereas all other group 8 elements have two electrons in the outermost shell, in ruthenium, the outermost shell has only one electron (the final electron is in a lower shell). This anomaly is observed in the neighboring metals niobium (41), molybdenum (42), and rhodium (45).

Ruthenium has four crystal modifications and does not tarnish at ambient conditions; it oxidizes upon heating to 800 °C (1,070 K). Ruthenium dissolves in fused alkalis to give ruthenates (RuO2−
), is not attacked by acids (even aqua regia) but is attacked by halogens at high temperatures.[6] Indeed, ruthenium is most readily attacked by oxidizing agents.[7] Small amounts of ruthenium can increase the hardness of platinum and palladium. The corrosion resistance of titanium is increased markedly by the addition of a small amount of ruthenium.[6] The metal can be plated by electroplating and by thermal decomposition. A ruthenium-molybdenum alloy is known to be superconductive at temperatures below 10.6 K.[6] Ruthenium is the last of the 4d transition metals that can assume the group oxidation state +8, and even then it is less stable there than the heavier congener osmium: this is the first group from the left of the table where the second and third-row transition metals display notable differences in chemical behavior. Like iron but unlike osmium, ruthenium can form aqueous cations in its lower oxidation states of +2 and +3.[8]

Ruthenium is the first in a downward trend in the melting and boiling points and atomization enthalpy in the 4d transition metals after the maximum seen at molybdenum, because the 4d subshell is more than half full and the electrons are contributing less to metallic bonding. (Technetium, the previous element, has an exceptionally low value that is off the trend due to its half-filled [Kr]4d55s2 configuration, though it is not as far off the trend in the 4d series as manganese in the 3d transition series.)[9] Unlike the lighter congener iron, ruthenium is paramagnetic at room temperature, as iron also is above its Curie point.[10]

The reduction potentials in acidic aqueous solution for some common ruthenium ions are shown below:[11]

0.455 V Ru2+ + 2e ↔ Ru
0.249 V Ru3+ + e ↔ Ru2+
1.120 V RuO2 + 4H+ + 2e ↔ Ru2+ + 2H2O
1.563 V RuO2−
+ 8H+ + 4e
↔ Ru2+ + 4H2O
1.368 V RuO
+ 8H+ + 5e
↔ Ru2+ + 4H2O
1.387 V RuO4 + 4H+ + 4e ↔ RuO2 + 2H2O


Naturally occurring ruthenium is composed of seven stable isotopes. Additionally, 34 radioactive isotopes have been discovered. Of these radioisotopes, the most stable are 106Ru with a half-life of 373.59 days, 103Ru with a half-life of 39.26 days and 97Ru with a half-life of 2.9 days.[12][13]

Fifteen other radioisotopes have been characterized with atomic weights ranging from 89.93 u (90Ru) to 114.928 u (115Ru). Most of these have half-lives that are less than five minutes except 95Ru (half-life: 1.643 hours) and 105Ru (half-life: 4.44 hours).[12][13]

The primary decay mode before the most abundant isotope, 102Ru, is electron capture and the primary mode after is beta emission. The primary decay product before 102Ru is technetium and the primary decay product after is rhodium.[12][13]


As the 74th most abundant element in Earth's crust, ruthenium is relatively rare,[14] found in about 100 parts per trillion.[15] This element is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario, Canada, and in pyroxenite deposits in South Africa. The native form of ruthenium is a very rare mineral (Ir replaces part of Ru in its structure).[16][17]