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Lanthanides & Actinides
Naturally Occurring Actinides
  • Only Actinium, Thorium, Protactinium & Uranium occur naturally (i.e. Z £ 92)
    • Actinium & Protactinium occur only in trace amounts
    • Neptunium & Plutonium occur in uranium minerals in minute amounts

      ~ not appreciated until after they had been synthesised that the synthesis route might occur naturally!

  • All isotopes of all the actinides are radioactive
  • Most of the longer-lived isotopes decay by a-emission
  • Half-lives Þ Only quantities of 232Th, 235U, 238U {and possibly 244Pu} could have survived since the formation of the solar system
  • Both Thorium and Uranium are far from rare


    • Widely dispersed, accounts for >3 ppm of the earth's crust
    • Natural Thorium is essentially 100% 232Th
    • Occurs in monazite [with the rare earths] and in uranothorite [a mixed Th,U silicate]
    • Obtained as ThO2, thoria, from mineral extraction process
    • Used as 99% ThO2 / 1% CeO2 in thoria gas mantles


    • Widely distributed - found scattered in the faults of old igneous rocks
    • Natural Uranium is 99.27% 238U & 0.72% 235U
    • Obtained usually as UO2
    • Used for nuclear fuel, and on a smaller scale for colouring glass/ceramics

  • Basic Features of Nuclear Structure & Chemistry and Radiochemistry
    • excellent, straightforward introduction: Sharpe, Inorganic Chemistry, Chapter 1
    • aspects of nuclear structure and stability: P.A. Cox, The Elements, OUP, 1989, Chapter 2 
  • 3 Decay series explain features of the occurrence, distribution & discovery of actinide elements

    Uranium Decay Series (from 238U)

Actinium Decay Series (from 235U)
Thorium Decay Series (from 232Th)
  • All Radon isotopes are short half-life a-emitters (but give rise to short-lived b-emitters)
    Radon gas is derived from Thorium content in Granite minerals Þ hazard in igneous areas
  • Actinium and Protactinium occur in uranium ores in trace amounts, because of their participation in Actinium Decay series (from 235U)
    In fact Protactinium was originally found by Fajans & Göhring (1913) as Brevium, 234Pa (T1/2 = 6.8 h), in the uranium (238U) decay series before Hahn & Meitner and Soddy & Cranston (1916) discovered the much longer-lived 231Pa (T1/2 = 33,000 y) in the actinium (235U) decay series. Named Protactinium because it is the parent of actinium in the decay series from 235U

--Info & DownloadsBibliography  [textbook & online resources]

Source: Dr. S.J. Heyes; University of Oxford
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