Lutetium (Lu)

• Element Symbol: Lu
• Atomic Number: 71
• Atomic Mass: 174.967
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: d-block (disputed)
• Discovered by: Georges Urbain (1907)

Lutetium is one of the rare earth metals. This group includes the fifteen elements in the lanthanide series as well as the elements ytterbium and scandium. The lanthanides are grouped together because they react chemically in ways that are similar to lanthanum’s behavior.

The term "rare earth metal" is a bit misleading because most of the elements in the group are comparatively plentiful in Earth’s crust. They are called rare because they are rarely found concentrated in ore deposits that are easy to mine and separate. Due to how lutetium and the other rare earth metals react chemically with other elements, they are usually found dispersed throughout Earth’s crust. They are typically found together and are so blended that it can be difficult to separate them from each other.

Lutetium is also considered a transition metal by some scientists. These thirty-eight metals are in groups 3 through 12 in the d-block at the center of the periodic table. All of the transition metals can combine chemically in a variety of ways. This is because the electrons available to form chemical bonds, called valence electrons, are located in more than one of the outer shells of their atoms. The best-known transition elements are iron, cobalt, and nickel.

Lutetium was the last natural rare earth metal to be discovered. It was identified as an impurity in samples of the mineral ytterbia. Researchers thought that the samples were mostly made up of another rare earth metal, ytterbium. Lutetium was discovered during three separate experiments in 1907. These experiments were performed by French scientist Georges Urbain, Austrian mineralogist Baron Carl Auer von Welsbach, and American chemist Charles James. Because he published his results first, George Urbain was given the right to name the element. He originally named it "lutecium," after Lutetia, the Latin word for the city of Paris. In 1949 the spelling was changed to lutetium. Because there was some dispute over who should be given credit for the discovery, the name Welsbach picked, cassiopeium, was used by some scientists until the 1950s.

Physical Properties

In its pure form lutetium is a silvery-white, hard, dense metal with hexagonal close-packed crystals. Because it reacts easily with many other elements, lutetium is never found in a free elemental state in nature. It is solid in its standard state—that is, its state at 298 kelvins (K).

The lutetium atom is the smallest among the atoms in the lanthanide series. The arrangement of the electrons in its outer shells makes possible a process called lanthanide contraction. This contraction causes the radius of a lutetium atom to be smaller than would be expected given the atom’s mass and composition. This small size helps explain why lutetium is the densest and hardest of the rare earth metals. Its density is 9.84 grams per cubic centimeter (g/cm³). Its hardness has been measured on the Brinell scale in the range of 890 to 1300 megapascals (mPa). At 1663 degrees Celsius (°C), it also has the highest meting point. Its boiling point is 3403 °C. It resists corrosion in dry air but will slowly tarnish in moist air.

Lutetium is paramagnetic over a wide range of temperatures. This means that it is only weakly attracted by a magnetic field. Like many metals, it can be made into a superconductor at high pressure and extremely low temperatures close to 0 K.

Chemical Properties

When lutetium forms chemical bonds, it loses three electrons from two different levels in its outer shell. This loss leaves it in an oxidation state of +3. Lutetium will react with the four lightest halogens—fluorine (F), chlorine (Cl), bromine (Br), and iodine (I)—to form trihalides. With the exception of the fluoride, all of these compounds are soluble in water. Most of lutetium’s salts are colorless. Lutetium dissolves readily in weak acids. It oxidizes at 150 °C to form lutetium oxide.

Most of the atoms of lutetium found on Earth are of its stable isotope, lutetium-175, which makes up 97.4 percent of the samples. The remaining 2.4 percent of the atoms are of its other naturally occurring isotope, lutetium-176. This radioactive isotope decays by beta emission into an isotope of hafnium. Lutetium-176 has a half-life of 3.78 × 10¹⁰ years. So far, thirty-two other radioactive isotopes of lutetium have been synthesized. Their atomic masses range from 150 to 184. The majority of these isotopes have half-lives that are less than thirty minutes.

Applications

With only 8 × 10⁻¹ milligrams per kilogram (mg/kg) of lutetium found in Earth’s crust and only 1.5 × 10⁻⁷ milligrams per liter (mg/L) showing up in the ocean, this element is one of the least plentiful of the lanthanides. Nonetheless, it still remains more plentiful than either silver or gold on Earth. Only one part per billion, by weight, of the universe is made up of lutetium.

Today, lutetium is primarily obtained from sands containing the mineral monazite through either an ion exchange process or by solvent extraction. A sample of pure lutetium metal itself was not made until 1953. Because lutetium is so difficult to locate, mine, and separate into its pure form, it is one of the most expensive metals. It can cost about six times as much per gram as gold. Due to these high costs, lutetium has few widespread practical uses. It is used in alloys, it acts as a catalyst in cracking hydrocarbons in petroleum, and it plays a role in alkylation, hydrogenation, and polymerization processes. Compounds containing lutetium are used in lenses for immersion lithography, in detectors in positron emission tomography (PET), and in magnetic bubble memory devices in computers. The element’s radioactive isotopes have been used to help date the age of meteorites, to treat cancerous tumors, and to reduce the pain caused by cancer. Lutetium oxide is known to absorb water and carbon dioxide. This ability makes it useful for removing vapors of these compounds from closed atmospheres.

Lutetium plays no active role in the biology of plants or animals, but some believe it may stimulate metabolism. It is not considered toxic, but its salts should be handled carefully.

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