Terbium is an element with the symbol Tb and the atomic number 65. It is a hard metal that is about as reactive as magnesium, but is corrosion resistant in air. It is classified as a lanthanide and a rare earth element. Due to its high reactivity, it does not exist as a free metal on Earth, but occurs with other rare earth elements in the minerals monazite, gadolinite, cerite, xenotime, bastnäsite and euxenite.
Alloys of terbium are often exhibit magnetostriction, or a change of shape in different magnetic fields. An alloy of terbium, iron, and dysprosium known as Terfenol-D is used in the SoundBug device to turn any solid surface into a speaker. Terbium ions exhibit both strong paramagnetism and bright green fluorescence, and terbium doped phosphors are used in cathode-ray tube televisions to produce green light, along with europium doped phosphors that produce red and blue.
Terbium is one of the few rare earths that exhibits multiple air-stable oxidation states.
Terbium is a dark silvery metal that is solid at room temperature. It is quite resistant to impact, but can be scratched with a knife or file.
At room temperature, terbium is paramagnetic and can be picked up easily with a neodymium magnet. This paramagnetism turns to antiferromagnetism at 230 K and to ferromagnetism at 219 K, which is above the temperature of dry ice. This causes a noticeable incrase in magnetic attraction. Terbium has the second-highest Curie temperature of the rare earth elements, exceeded only by gadolinium.
Terbium(III) cations fluoresce to produce bright green light. The fluorescence is normally only visible in the solid state, but certain ligands will allow solutions of these salts to fluoresce in solution. These ions are also paramagnetic, and compounds of terbium(III) may be lifted by a powerful magnet.
Although terbium is a highly electropositive element, the metal can be stored indefinitely in air and does not corrode, even after years of exposure. However, terbium reacts slowly in cold water, and vigorously in hot water and dilute acids to form trivalent salts.
Terbium reacts with all of the halogens to form trihalides. Small pieces can burn in air with a golden flame to form terbium(III,IV) oxide, which is black, and markedly different from terbium(III) oxide.
Terbium sulfate is only poorly soluble in water, and its solubility decreases as temperature increases. Terbium oxalate is extremely insoluble in water.
Terbium(III) is the most stable form of the element. It is not amphoteric, and adding bases such as ammonia to terbium-containing solutions will produce a precipitate of white terbium hydroxide. This hydroxide is itself basic, and will absorb carbon dioxide to form terbium carbonate. This change is not very noticeable.
Terbium(IV) compounds are highly oxidizing. Terbium(III,IV) oxide is a black powder which is similar to manganese dioxide. It is also an excellent catalyst. Terbium(IV) fluoride is one of the best fluorinating agents known, due to its ability to emit relatively pure atomic fluorine, rather than fluoride vapors.
Toxicity data of terbium compounds is scarce, but they appear to be of low to moderate danger. Terbium plays no biological role, but acts similarly to calcium within the body. Some terbium compounds will hydrolyze when heated and will give off acidic vapors.
Small pieces of terbium metal or terbium powder are flammable. Class D fire extinguishers should be readily available when working with terbium near an open flame.