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Tantalum: structure, properties, uses, obtaining

Tantalum was discovered in 1802 by the Swedish chemist Anders Gustaf Ekenberg, naming it in memory of the Greek mythological character Tantalus, relating it to the tempting problem of dissolving metal oxides in acid.

Metallic fragments of tantalum. Source: Hi-Res Images of Chemical Elements, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

However, because tantalum and niobium are very similar metals, Ekenberg was thought to have isolated niobium. But in 1844 the German chemist Heinrich Rose proved that niobium and tantalum are distinct metals, and credit was returned to Ekenberg for their discovery.

Tantalum is a metal whose main utility lies in the manufacture of small capacitors used in cell phones, laptops, electronic cameras, etc. Also, tantalum is used for medical purposes in bone implants and in the repair of skull bones.

Tantalum structure

Tantalum can adopt two crystalline structures at room temperature: the cubic one centered on the body (bcc), designated as phase α (α-Ta) and responsible for its ductility; and the tetragonal or β phase (β-Ta), which is metastable, contributes hardness, and transforms into the α phase when the crystals are heated between 750-775ºC.

Some sources also report the existence of a third crystalline structure: the face-centered cubic (fcc), which is considered anomalous and has only been observed in very thin tantalum sheets. Thus, this structure (fcc-Ta) counts as a third allotrope.

Electronic configuration

Tantalum has the following electronic configuration:

[Xe] 4f 14 5d 3 6s 2

The electrons in the 4f orbital are very “inside” the tantalum atom in terms of energy; that is, they do not participate in any of its chemical reactions. Therefore, tantalum can only use the electrons of the 5d and 6s orbitals to lose them and transform into cations, so that they interact with other atoms or anions to originate their compounds.

When tantalum loses the five electrons from these 5d and 6s orbitals, it becomes the pentavalent cation Ta 5+ , which is very electronically stable; characteristic that in fact it shares with its counterparts V 5+ and Mo 5+ . This is why tantalum (V) or +5 compounds are the most common for this metal.

Physical appearance

It is a shiny but dark metal, and it can present some bluish or purple overtones. These colorations are due to the layers with different thicknesses of its oxide that cover the surface of the metal.

Tantalum pieces are very ductile and malleable, although depending on their production process they can also be very hard and brittle.

It is a good conductor of electricity and heat. It has a very high melting point (3017 ºC), only surpassed by tungsten, rhenium and osmium. Furthermore, it is one of the few refractory metals; that is, it is very resistant to heat and wear.

Tantalum has two allotropic forms: alpha, ductile and soft; and the beta, hard and brittle. It is a metal resistant to corrosion, due to an oxide film that covers it. Tantalum in powder form can spontaneously ignite in air.

Atomic number

73

Molar mass

180.947 g / mol

Melting point

3017 ºC

Boiling point

5458 ºC

Density

16.69 g / cm 3

Heat of fusion

36.57 kJ / mol

Heat of vaporization

753 kJ / mol

Molar caloric capacity

25.36 J / (mol K)

ignition point

> 250 ºC

Refractive index

2.05

Hardness

6.5 on the Mohs scale

Oxidation states

Tantalum atoms can participate in their compounds with the following oxidation states: -3, -1, +1, +2, +3, +4 and +5, the latter being the most predominant. Therefore, tantalum (V) or +5 (Ta 5+ ) compounds are more common and stable .

Electronegativity

1.5 Pauling scale.

Ionization energies

First: 761 kJ / mol

Second: 1500 kJ / mol

Reactivity

Tantalum at temperatures below 150ºC is one of the metals with the greatest chemical inertness or the least reactivity. At room temperature it is not attacked by diluted or concentrated acids, it is even resistant to the action of aqua regia. It can only be dissolved by hydrofluoric acid.

Tantalum is not attacked by most alkalis dissolved in water, but it is attacked by molten alkalis and potassium hydroxide. At elevated temperatures it can react with fluorine, chlorine and oxygen.

Tantalum oxidizes to form a pentoxide (Ta 2 O 5 ), a compound that has several applications. Nitride and tantalum carbide are very hard compounds used in cutting tools. In addition, tantalum can be part of some organometallic compounds, such as: pentamethyl tantalum, Ta (CH 3 ) 5 .

Applications

Parts used in electricity and electronics

Tantalum capacitor in an electronic circuit. Source: Epop, Public domain, via Wikimedia Commons

Capacitors or capacitors built with tantalum have a higher value of the relationship between capacitance and volume than any other type of capacitor, thus allowing them to be used in instruments that have microcircuits; such as cell phones, laptops, electronic cameras, etc.

Tantalum has been used in the production of ultra-high frequency electron tubes for radio transmitters. Tantalum reacts with nitrogen and oxygen helping to maintain the high vacuum in the tubes, necessary when using internal parts such as grids and plates.

It is also used to manufacture electrodes for neon lights and AC / DC rectifiers. Tantalum, due to its ductility, is used to make very thin wires that are used in the evaporation of aluminum.

High hardness parts

Tantalum is a hard and resistant metal, which is why it has been used, in the form of carbide, in cutting tools and metalworking. It has also been used in the manufacture of jet engines, nuclear reactors, missile parts, turbine blades, and nose caps for aircraft.

High chemical resistance items

Because tantalum is a metal that is inert to a large number of chemical compounds, and also resistant to corrosion, it is used in vessels for chemical reactions, in the construction of pipes for corrosive liquids, as well as in laboratory equipment.

In addition, due to its high melting point and resistance to oxidation, it is used in the manufacture of crucibles for distillation in vacuum furnaces.

Medical uses

Tantalum is a non-irritating metal and resistant to body fluids, which is why it has been used in orthopedic bone implants; and furthermore, due to its malleability, metal sheets used in the repair of damaged portions of the skull have been constructed. Said sheets are also used in the connection of torn ribs.

Tantalum is used in the production of trabecular metal, which is similar to a bone material and approximates its physical properties. Its trabecular and porous configuration favors bone formation and rapid and extensive tissue infiltration.

This procedure has application in hip and knee arthroplasty, and in the treatment of necrosis of the bone tissue of the femoral head.

Hyaluronan is a gel used to transport medications through a catheter percutaneously. The tantalum is incorporated into the gel to make it opaque to X-rays and to observe its entry into the body.

Also, tantalum has been investigated for the prevention of osteoarthritis and adolescent idiopathic scoliosis.

Others

Tantalum is used in the rubber industry as a catalyst in the synthesis of butadiene. It is also used in the manufacture of pen tips, analytical balances, and surgical and dental instruments to replace platinum.

Tantalum oxide is used in the manufacture of a high refractive index glass for camera lenses.

Obtaining

Tantalum has an abundance between 1 and 2 ppm in the earth’s crust, not being in a free state. It is exploited commercially from the mineral tantalite, although it is also found in columbite and coltan.

The mineral to be crushed, is leached with hydrofluoric acid mixed with sulfuric or hydrochloric acid, producing a complex of tantalum and fluoride, H 2 [TaF 7 ]. This complex is subjected to a liquid-liquid extraction using organic solvents.

2 [TaF 7 ] is treated with potassium fluoride to produce potassium fluorotantalate, K 2 [TaF 7 ], which is finally reduced with metallic sodium by heating in an electric furnace, thus obtaining metallic tantalum.

Isotopes

Tantalum has a total of 38 isotopes, of which only one can be considered stable: 181 Ta. This isotope has an abundance of 99.98% of tantalum, while the remaining 0.12% is represented by 180m Ta.

The isotope with the shortest half-life is 191 Ta, with a value greater than 300 nanoseconds; while the one with the longest half-life is the aforementioned 180m Ta 2.0 · 10 16  . Part of the radioactive decay is produced by electronic capture and by emission of α and β  particles .

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