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Francium-223 is the most stable isotope, with a half-life of 21.8 minutes, [8] and it is highly unlikely that an isotope of francium with a longer half-life will ever be discovered or synthesized. [22] Francium-223 is a fifth product of the uranium-235 decay series as a daughter isotope of actinium-227; thorium-227 is the more common daughter. [23]
The extremely rare elements technetium and promethium can be found in uraninite in very small quantities (about 200 pg/kg and 4 fg/kg respectively), produced by the spontaneous fission of uranium-238. Francium can also be found in uraninite at 1 francium atom for every 1 × 10 18 uranium atoms in the ore as a result from the decay of actinium.
Fr: Francium: 1.87 ~ 1×10 −18 [ax] (2.77 × 10 −2 kg) Not traded. [cr] 88: Ra: Radium: 5.5: 9×10 −7 [ax] (2.493 × 10 10 kg) Negative price. [cs] 89: 225 Ac: Actinium-225: 10.07: 29 × 10 12: 290 × 10 12: 2004 [ay] CRC Handbook (ORNL) [co] 90: Th: Thorium: 11.72: 9.6 (2.6592 × 10 17 kg) 287: 3360: 2010: USGS MYB 2012 [63] [ct] 91: Pa ...
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.463 × 10 9 years, [7] roughly the age of the Earth. Uranium-238 is predominantly an alpha emitter, decaying to thorium-234. It ultimately decays through the uranium series, which has 18 members, into lead-206. [17]
Uranium-235 (235 U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nature as a primordial nuclide. Uranium-235 has a half-life of 703.8 million years.
The three long-lived nuclides are uranium-238 (half-life 4.5 billion years), uranium-235 (half-life 700 million years) and thorium-232 (half-life 14 billion years). The fourth chain has no such long-lasting bottleneck nuclide near the top, so almost all of the nuclides in that chain have long since decayed down to just before the end: bismuth-209.
A synthetic element is one of 24 known chemical elements that do not occur naturally on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; thus, they are called "synthetic", "artificial", or "man-made".
This is especially problematic for francium, which by relativistic calculations can be shown to be less electronegative than caesium, but for which the only value (0.7) in the literature predates these calculations.