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 increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding. At the start of period 7, from francium to thorium, the melting points of the elements increase (as in other periods), because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. Thorium's melting point of 1750 ☌ is above both those of actinium (1227 ☌) and protactinium (1568 ☌). Thorium is nearly half as dense as uranium and plutonium and is harder than both. Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire. Aluminium's is 75.2 GPa copper's 137.8 GPa and mild steel's is 160–169 GPa.
Thorium metal has a bulk modulus (a measure of resistance to compression of a material) of 54 GPa, about the same as tin's (58.2 GPa). At room temperature, thorium metal has a face-centred cubic crystal structure it has two other forms, one at high temperature (over 1360 ☌ body-centred cubic) and one at high pressure (around 100 GPa body-centred tetragonal). Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn. In the periodic table, it lies to the right of actinium, to the left of protactinium, and below cerium. Thorium is a moderately soft, paramagnetic, bright silvery radioactive actinide metal. Ocean science has utilised 231 Pa/ 230Th isotope ratios to understand the ancient ocean. Other uses for thorium include heat-resistant ceramics, aircraft engines, and in light bulbs. Thorium is also used in strengthening magnesium, coating tungsten wire in electrical equipment, controlling the grain size of tungsten in electric lamps, high-temperature crucibles, and glasses including camera and scientific instrument lenses. It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, and several thorium reactors have been built. It was also material in high-end optics and scientific instrumentation, used in some broadcast vacuum tubes, and as the light source in gas mantles, but these uses have become marginal. Thorium is still being used as an alloying element in TIG welding electrodes but is slowly being replaced in the field with different compositions. In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity. Thorium's radioactivity was widely acknowledged during the first decades of the 20th century. Its first applications were developed in the late 19th century. Thorium was discovered in 1828 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder. Thorium is estimated to be over three times as abundant as uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals. On Earth, thorium and uranium are the only significantly radioactive elements that still occur naturally in large quantities as primordial elements.
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The most stable isotope, 232Th, has a half-life of 14.05 billion years, or about the age of the universe it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable 208 Pb. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state it is quite reactive and can ignite in air when finely divided.Īll known thorium isotopes are unstable. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide it is moderately soft and malleable and has a high melting point. Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90.
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