|Name, Symbol, Number||terbium, Tb, 65|
|Group, Period, Block||n/a, 6, f|
|Atomic mass||158.92535(2) g/mol|
|Electron configuration||[Xe] 4f9 6s2|
|Electrons per shell||2, 8, 18, 27, 8, 2|
|Density (near r.t.)||8.23 g/cm³|
|Liquid density at m.p.||7.65 g/cm³|
|Melting point||1629 K
(1356 °C, 2473 °F)
|Boiling point||3503 K
(3230 °C, 5846 °F)
|Heat of fusion||10.15 kJ/mol|
|Heat of vaporization||293 kJ/mol|
|Heat capacity||(25 °C) 28.91 J/(mol·K)|
|Oxidation states||3, 4
(weakly basic oxide)
|Electronegativity||? 1.2 (Pauling scale)|
|1st: 565.8 kJ/mol|
|2nd: 1110 kJ/mol|
|3rd: 2114 kJ/mol|
|Atomic radius||175 pm|
|Atomic radius (calc.)||225 pm|
in dry ice 
|Electrical resistivity||(r.t.) (α, poly)
|Thermal conductivity||(300 K) 11.1 W/(m·K)|
|Thermal expansion||(r.t.) (α, poly)
|Speed of sound (thin rod)||(20 °C) 2620 m/s|
|Speed of sound (thin rod)||(r.t.) (α form) 55.7 m/s|
|Shear modulus||(α form) 22.1 GPa|
|Bulk modulus||(α form) 38.7 GPa|
|Poisson ratio||(α form) 0.261|
|Vickers hardness||863 MPa|
|Brinell hardness||677 MPa|
|CAS registry number||7440-27-9|
Terbium is never found in nature as the free element, but it is contained in many minerals. For instance, it occurs in cerite, gadolinite, monazite ((Ce,La,Th,Nd,Y)PO4, which contains up to 0.03 percent of terbium), xenotime (YPO4) and euxenite ((Y,Ca,Er,La,Ce,U,Th)(Nb,Ta,Ti)2O6, which contains one percent or more of terbium).
Terbium was discovered in 1843 by Swedish chemist Carl Gustaf Mosander, who detected it as an impurity in Yttrium-oxide, Y2O3, and named after the village Ytterby in Sweden. It was not isolated in pure form until the recent advent of ion exchange techniques.
Terbium is classified as a rare earth element. The term "rare" is misleading because terbium is more common than metals such as silver and mercury. The name "rare earth" meant something else to early chemists. It was used because the rare earth elements were very difficult to separate from each other. They were not "rare" in the Earth, but they were "rarely" used for anything.
Terbium is an inner transition metal (or lanthanide) that lies in period six of the periodic table, between gadolinium and dysprosium. It is malleable, ductile, and soft enough to be cut with a knife. It is reasonably stable in air, and two crystal allotropes exist, with a transformation temperature of 1,289 °C.
Naturally occurring terbium is composed of one stable isotope, 159-Tb. 33 radioisotopes have been characterized, with the most stable being 158-Tb with a half-life of 180 years, 157-Tb with a half-life of 71 years, and 160-Tb with a half-life of 72.3 days. All of the remaining radioactive isotopes have half-lifes that are less than 6.907 days, and the majority of these have half lifes that are less than 24 seconds. This element also has 18 meta states, with the most stable being 156m1-Tb (t½ 24.4 hours), 154m2-Tb (t½ 22.7 hours) and 154m1-Tb (t½ 9.4 hours).
The primary decay mode before the most abundant stable isotope, 159-Tb, is electron capture, and the primary mode after is beta minus decay. The primary decay products before 159-Tb are element Gd (gadolinium) isotopes, and the primary products after are element Dy (dysprosium) isotopes.
Terbium compounds include:
See also terbium compounds.
As with the other lanthanides, terbium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail. Terbium has no known biological role.
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