Thorium

actinium ← thorium → protactinium Ce ↑ Th ↓ —

Atomic properties Atomic number 90 Standard atomic weight 232.038 06(2) Electron configuration [Rn] 6d2 7s2 Physical properties Melting point 1750 °C (2023 K) Boiling point 4850 °C (5123 K) Density 11.72 g cm−3 Electric resistivity 15.7 × 10−6 Ω cm Ionization energy 6.3067(2) eV 608.50(2) kJ mol−1 Atomic radii Covalent radius 206 pm Metallic radius 180 pm Ionic radius 108 pm (Th4+, Oh) Thermodynamic properties Standard entropy 51.8(5) J K−1 mol−1 Enthalpy change of fusion 14 kJ mol−1 Enthalpy change of vaporization 602(6) kJ mol−1 Miscellaneous CAS number 7440-29-1 EC number 231-139-7 Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).

Thorium (symbol: Th) is a member of the actinoid series of chemical elements. Although all isotopes of thorium are unstable, thorium-232 has a sufficiently long half-life (14 billion years) that a substantial amount of primordial thorium has survived since the formation of the Solar System. The natural material is only very slightly radioactive, and thorium and its compounds have a number of commercial uses, although some precautions must be taken over its decay products.

Thorium was identified as a new element in 1828, and named after Thor, the Norse god of strength. Its chemistry resembles that of zirconium and hafnium, with a preponderance of the +4 oxidation state, and periodic tables from before the Second World War often placed thorium as a transition metal.

Discovery and history

Thorium was first identified in a sample of a new mineral (now called thorite) from the island of Løvøy in Vestfold, Norway. The sample was collected by the Lutheran pastor and amateur mineralogist Hans Morten Thrane Esmark; unable to identify it, he sent it to Christiana (modern-day Oslo) where his father Jens Esmark was professor of mineralogy and geology at the university. When Esmark Sr. had no more success than his son, he passed the sample on to the reknowned Swedish chemist Jöns Jacob Berzelius, who correctly ascertained that the mineral contained a new chemical element.

Thorium remained something of a curiosity until the invention of the incandescent gas mantle in 1885 by Carl Auer von Welsbach. The cover of thorium dioxide (with 1% cerium dioxide) over and around the gas flame increased the light output tenfold through candoluminescence, and led to a huge expansion of public lighting in the late nineteenth and early twentieth centuries.

External links

• WebElements