Thorium is a weakly radioactive metallic 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.
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. The most stable isotope, It is estimated to be over three times more abundant than uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals.
Thorium was discovered in 1829 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.
Its first applications were developed in the late 19th century.
Thorium's radioactivity was widely acknowledged during the first decades of the 20th century.
In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity.
Thorium is a moderately soft, 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.Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn.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).Aluminium's is 75.2 GPa; copper's 137.8 GPa; and mild steel's is 160–169 GPa.Thorium's melting point of 1750 °C is above both those of actinium (1227 °C) and protactinium (1568 °C).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, 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. 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.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).