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In the beginning 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.After thorium, there is a new downward trend in melting points from thorium to plutonium.It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, and several thorium reactors have been built.Thorium is a moderately hard, paramagnetic, bright silvery radioactive actinide metal.All but two elements up to bismuth (element 83) have an isotope that is practically stable for all purposes ("classically stable"), with the exceptions being technetium and promethium (elements 43 and 61).All elements from polonium (element 84) onward are measurably radioactive.Its first applications were developed in the late 19th century.Thorium's radioactivity was widely acknowledged during the first decades of the 20th century.
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At room temperature, thorium metal has a face-centred cubic crystal structure; it has two other forms, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).
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).
Thorium is a chemical element with symbol Th and atomic number 90.
Thorium metal is silvery and tarnishes black when it is exposed to air, forming the dioxide; it is moderately hard, malleable, and has a high melting point.