Lutetium is a chemical element with the symbol Lu and atomic number 71. It is a silvery white metal that is relatively rare in nature. Lutetium is the last element in the lanthanide series, and it is traditionally counted among the rare earth elements. It is also sometimes considered the first element of the 6th-period transition metals. Lutetium is used in various applications, such as catalysts, radiation detectors, and X-ray machines. It is also used in some medical treatments, such as cancer therapy.