Lutetium ( ) is a chemical element with the symbol Lu and atomic number 71. It is the last element in the lanthanide series, which, along with the lanthanide contraction, explains several important properties of lutetium, such as it having the highest hardness or density among lanthanides. Unlike other lanthanides, which lie in the f-block of the periodic table, this element lies in the d-block; however, lanthanum is sometimes placed on the d-block lanthanide position. Chemically, lutetium is a typical lanthanide: its only common oxidation state is +3, seen in its oxide, halides and other compounds. In an aqueous solution, like compounds of other late lanthanides, soluble lutetium compounds form a complex with nine water molecules.