Tantalum

Tantalum is a chemical element with symbol Ta and atomic number 73. Previously known as tantalium, its name comes from Tantalus, an antihero from Greek mythology Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion-resistant. It is part of the refractory metals group, which are widely used as minor components in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum. Tantalum is also used for medical implants and bone repair Its main use today is in tantalum capacitors in electronic equipment such as mobile phones, DVD players, video game systems and computers. Tantalum, always together with the chemically similar niobium, occurs in the minerals tantalite, columbite and coltan (a mix of columbite and tantalite).

Tantalum is a rare metal. Its abundance in the solar system can be estimated via spectroscopic study of the sun and chemical analyses of meteorites. In the solar system abundance list used in Wikipedia, its molar concentration relative to silicon is reported to be either 2.1×10⁻⁸ (Ahrens) or 3.8x10^-8 (Kaye & Laby). Converted to fraction of all atoms in the primordial solar mix, [Ta] = 1.3E-12 per unit. In abundance, it ranks just above uranium at the tail end of the primordial elements list. A second table lists a whole-earth concentration of [Ta] = 4000E-12 (4E-09) per unit. This enhanced concentration of Ta occurs because He and Ne have essentially disappeared from the earth, and both H and C have terrestrial abundances much lower than their solar abundances. Its concentration in the crust, however is reported to be around 2 parts per million by mass (ppm). Since its atomic mass is several times earth's mean, its atomic (molar) concentration will be several times lower so [Ta] = 5.00E-09 per unit is a reasonable guess. Tantalum is somewhat concentrated in the crust, but not to the extent of U or other active metals.

Nuclear properties

36 isotopes of Ta have been observed, ranging from ¹⁹⁰Ta down to ¹⁵⁵Ta. These lie within the band of predicted isotopes, which ranges from ²⁴⁷Ta to ¹⁵⁴Ta. In addition, 37 isomers have been observed, 26 of them in the band ¹⁸²Ta to ¹⁷⁶Ta and 7 of them in the band ¹⁶⁰Ta to ¹⁵⁶Ta. It is not clear whether this concentration results from nuclear structure or limited search for isomers outside the bands given.

¹⁸¹Ta is the element's only stable isotope, although ¹⁷⁹Ta (half-life 1.82 y) comes fairly close. Between them lies ¹⁸⁰Ta, which has an 8.15 hr half-life, decays both by electron capture to ¹⁸⁰Hf (branch ratio = 0.86) and by electron emission to ¹⁸⁰W (BR = 0.14), and has a metastable isomer. That isomer ^180m1Ta is unique. It is a high-spin isomer whose decay to its ground state (¹⁸⁰Ta) is so strongly inhibited that it has never been observed. Since its decay by any other mode consumes energy, it never decays. It is the one and only stable isomer.

Above ¹⁸¹Ta, isotopes decay by beta emission with short half-lives. Below ¹⁷⁹Ta, all isotopes decay by positron emission except for ¹⁵⁵Ta whose reported half-life is too short for beta decay and so is predicted to decay by proton emission. Alpha decay is an active decay mode between ¹⁶³Ta and ¹⁵⁷Ta, for which N = 84, at which point alpha decay is replaced by some proton emission in ¹⁵⁶Ta and near-total proton emission in ¹⁵⁵Ta and ¹⁵⁴Ta.

¹⁸¹Ta accounts for 0.99988 +/- 0.00002 of all Ta atoms. ^180m1Ta provides the remaining 0.00012 +/- 0.00002 atoms. While ¹⁸¹Ta forms via either rapid neutron capture (r-process) or infrequent neutron capture (s-process), neither will produce ^180m1Ta. The most likely possible route would be photon capture / neutron emission: ¹⁸¹Ta + gamma --> ^180m1Ta + n.