Terbium is reasonably stable in air. It is a silvery-grey metal, and is malleable, ductile, and soft enough to be cut with a knife. It is a rare earth metal found in cerite, gadolinite and monazite. The element itself was isolated only recently.
This sample is fromThe Elements Collection, an attractive and safely packaged collection of the 92 naturally occurring elements that is available for sale.
Isolation: terbium metal is available commercially so it is not normally necessary to make it in the laboratory, which is just as well as it is difficult to isolate as the pure metal. This is largely because of the way it is found in nature. The lanthanoids are found in nature in a number of minerals. The most important are xenotime, monazite, and bastnaesite. The first two are orthophosphate minerals LnPO4 (Ln deonotes a mixture of all the lanthanoids except promethium which is vanishingly rare) and the third is a fluoride carbonate LnCO3F. Lanthanoids with even atomic numbers are more common. The most comon lanthanoids in these minerals are, in order, cerium, lanthanum, neodymium, and praseodymium. Monazite also contains thorium and ytrrium which makes handling difficult since thorium and its decomposition products are radioactive.
For many purposes it is not particularly necessary to separate the metals, but if separation into individual metals is required, the process is complex. Initially, the metals are extracted as salts from the ores by extraction with sulphuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH). Modern purification techniques for these lanthanoid salt mixtures are ingenious and involve selective complexation techniques, solvent extractions, and ion exchange chromatography.
Pure terbium is available through the reduction of TbF3 with calcium metal.
2TbF3 + 3Ca → 2Tb + 3CaF2
This would work for the other calcium halides as well but the product CaF2 is easier to handle under the reaction conditions (heat to 50°C above the melting point of the element in an argon atmosphere). Excess calcium is removed from the reaction mixture under vacuum.
Density of solid: 8219 kg m-3
Molar volume: 19.30 cm3
Thermal conductivity: 11 W m‑1 K‑1
Melting point: 1629 [1356 °C (2473 °F)] K
Boiling point: 3503 [3230 °C (5846 °F)] K
Enthalpy of fusion: 20.5 kJ mol-1
Atomic radius (empirical): 175 pm
Molecular single bond covalent radius: 168 (coordination number 3) ppm
van der Waals radius: 287 ppm
Pauling electronegativity: (no data) (Pauling units)
Allred Rochow electronegativity: 1.10 (Pauling units)
Mulliken-Jaffe electronegativity: (no data)
First ionisation energy: 565.77 kJ mol‑1
Second ionisation energy: 1110.8 kJ mol‑1
Third ionisation energy: 2110 kJ mol‑1
Universe: 0.5 ppb by weight
Crustal rocks: 940 ppb by weight
Human: (no data) ppb by weight
