• 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

  Thorium

  • 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

  Uranium

  • 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 (T_1/2 = 6.8 h), in the uranium (²³⁸U) decay series before Hahn & Meitner and Soddy & Cranston (1916) discovered the much longer-lived ²³¹Pa (T_1/2 = 33,000 y) in the actinium (²³⁵U) decay series. Named Protactinium because it is the parent of actinium in the decay series from ²³⁵U

Bibliography