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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.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 over its radioactivity.
Among the actinides, thorium has the highest melting and boiling points and second-lowest density; actinium is lighter.
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.
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.
Uranium–thorium dating is commonly used to determine the age of calcium carbonate materials such as speleothem or coral, because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floor sediments, where their ratios are measured.