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, comprising 6991800000000000000♠8×10−9% of the universe, making it one-fifteenth as abundant in the universe as gold (which makes up 6992600000000000000♠6×10−8%). Tantalum also comprises 6996150000000000000♠1.5×10−4% of the earth's crust, making it more abundant than other metals in the sixth period, such as rhenium (abundance 6993260000000000000♠2.6×10−7%), osmium (abundance 6993180000000000000♠1.8×10−7%), and iridium (abundance 6992400000000000000♠4×10−8%), but not as abundant as barium (abundance 6998340000000000000♠3.4×10−2%).
Isotopes of tantalum
Natural tantalum (Ta) consists of two stable isotopes: 181Ta (99.988%) and 180mTa (0.012%).
The latter nuclide 180mTa (m denotes a metastable state) has sufficient energy to decay in three ways: isomeric transition to the ground state of 180Ta, beta decay to 180W, and electron capture to 180Hf. However, no radioactivity from any decay mode of this nuclear isomer has ever been observed. Only a lower limit on its half-life of over 1015 years has been set, by observation. The very slow decay of 180mTa is attributed to its high spin (9 units) and the low spin of lower-lying states. Gamma or beta decay would require many units of angular momentum to be removed in a single step, so that the process would be very slow.
The very unusual nature of 180mTa is that the ground state of this isotope is less stable than the isomer. The same property is exhibited in americium-242m (242mAm). 180Ta has a half-life of only 8 hours. 180mTa is the only naturally occurring nuclear isomer (excluding radiogenic and cosmogenic short-living nuclides). It is also the rarest primordial nuclide in the Universe observed for any element that has any stable isotopes.
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