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Th decay initiates the 4n decay chain which includes isotopes with a mass number divisible by 4 (hence the name; it is also called the thorium series after its progenitor).
This chain of consecutive alpha and beta decays begins with the decay of Th also very occasionally undergoes spontaneous fission rather than alpha decay, and has left evidence of doing so in its minerals (as trapped xenon gas formed as a fission product), but the partial half-life of this process is very large at over 10Np: the last of these is long extinct in nature due to its short half-life (2.14 million years), but is continually produced in minute traces from neutron capture in uranium ores.
Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377(4).
It is one of only three radioactive elements (along with protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.
After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f electrons increases from about 0.4 to about 6: this trend is due to the itinerance of the f-orbitals, increasing hybridisation of the 5f and 6d orbitals, and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding. Addition of small proportions of thorium improves the mechanical strength of magnesium, and thorium-aluminium alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors.
Thorium forms eutectic mixtures with chromium and uranium, and it is completely miscible in both solid and liquid states with its lighter congener cerium.
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The scheme has a range of several hundred thousand years.
In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity.
Thorium was formerly used as an alloying element in TIG welding electrodes, as a material in high-end optics and scientific instrumentation, and as the light source in gas mantles, but these have become marginal uses.
It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, and several thorium reactors have been built.
paramagnetic, bright silvery radioactive actinide metal. In the periodic table, it lies to the right of actinium, to the left of protactinium, and below cerium.
Thorium is a weakly radioactive metallic chemical element with symbol Th and atomic number 90.