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This article is about the chemical element. For other uses, see Cobalt (disambiguation).– ↑Co ↓Rhiron ← cobalt → nickelAtomic number27Standard atomic weight (±)58.933194(4)Element category  transition metalGroup, blockgroup 9, d-blockPeriodperiod 4Electron configuration[Ar] 3d 4sper shell2, 8, 15, 2Physical propertiesColormetallic grayPhasesolidMelting point1768  ​(1495 °C, ​2723 °F)Boiling point3200 K ​(2927 °C, ​5301 °F)Density near r.t.8.90 g·cmwhen liquid, at m.p.8.86 g·cmHeat of fusion16.06 kJ·molHeat of vaporization377 kJ·molMolar heat capacity24.81 J·mol·Kvapor pressureAtomic propertiesOxidation states−1, +1, +2, +3, +4, +5 ​(an amphoteric oxide)ElectronegativityPauling scale: 1.88Ionization energies1st: 760.4 kJ·mol 2nd: 1648 kJ·mol 3rd: 3232 kJ·mol (more)Atomic radiusempirical: 125 pmCovalent radiusLow spin: 126±3 pm High spin: 150±7 pmMiscellaneaCrystal structure​hexagonal close-packed (hcp) Speed of sound thin rod4720 m·s (at 20 °C)Thermal expansion13.0 µm·m·K (at 25 °C)Thermal conductivity100 W·m·KElectrical resistivity62.4 nΩ·m (at 20 °C)Magnetic orderingferromagneticYoung's modulus209 GPaShear modulus75 GPaBulk modulus180 GPaPoisson ratio0.31Mohs hardness5.0Vickers hardness1043 MPaBrinell hardness470–3000 MPaCAS Registry Number7440-48-4HistoryDiscoveryGeorg Brandt (1732)Most stable isotopesMain article: Isotopes of cobalt