Tantalum is synthesized in stars mainly through the s-process (slow neutron capture) occurring in low- to medium-mass AGB stars (asymptotic giant branch), with a significant contribution from the r-process (rapid neutron capture) during explosive events such as supernovae. As a heavy element with an odd atomic number (Z=73), it is less abundant than its even-numbered neighbors (hafnium-72 and tungsten-74) according to the Oddo-Harkins rule. Tantalum-181, its only stable natural isotope, is mainly produced by the s-process, but some short-lived radioactive isotopes of tantalum are produced exclusively by the r-process.
The cosmic abundance of tantalum is about 8.0×10⁻¹³ times that of hydrogen in number of atoms, making it about twice as rare as hafnium and one of the rarest heavy elements. In the solar system, tantalum has an abundance similar to that of gold and platinum. Tantalum-181 is the only stable natural isotope, accounting for 99.988% of natural tantalum, with tantalum-180m (metastable isomer) making up the remaining 0.012%.
Detecting tantalum in stellar atmospheres is extremely difficult due to its rarity. However, it has been detected in some s-process-rich stars through the spectral lines of Ta I and Ta II. The tantalum/hafnium (Ta/Hf) ratio in stars provides information on the conditions of neutron-capture nucleosynthesis, as these two neighboring elements have similar nuclear behaviors but different chemical properties that can affect their observation in stellar spectra.
Tantalum plays an important role in the tantalum-tungsten (Ta-W) dating system. Tantalum-182, a now-extinct radioactive isotope (half-life of 114.43 days), decays into tungsten-182. This chronometer system is crucial for dating very early events in the solar system, as tantalum and tungsten have very different geochemical behaviors during planetary core formation: tantalum is lithophile (prefers silicates) while tungsten is siderophile (prefers metal). Tungsten-182 anomalies in meteorites and lunar samples allow dating the formation of the Earth's core and the differentiation of planetary bodies in the first few million years of the solar system.
Tantalum is named after the Greek mythological figure Tantalus, king of Lydia punished by the gods for stealing their nectar and ambrosia. According to the myth, Tantalus was condemned to stand in a lake whose water receded when he tried to drink, and under a fruit tree whose branches rose when he tried to eat, leaving him in eternal thirst and hunger. The name was chosen by the discoverer Anders Gustaf Ekeberg to evoke the inability of tantalum oxide to absorb acids and dissolve, remaining "insatiable" like Tantalus in the myth.
Tantalum was discovered in 1802 by the Swedish chemist Anders Gustaf Ekeberg (1767-1813) at Uppsala University. Ekeberg was analyzing minerals from Sweden and Finland when he isolated a new oxide insoluble in acids. He named this oxide "tantalite" after the myth of Tantalus, and the corresponding element "tantalum". Ekeberg was already deaf at the time of his discovery, but this did not prevent him from making major contributions to mineral chemistry.
For several decades, tantalum was confused with another element discovered around the same time, niobium (then called columbium). In 1809, the English chemist William Hyde Wollaston declared that tantalum and columbium were the same element. It was not until 1846 that the German chemist Heinrich Rose demonstrated that they were two distinct elements, which he named niobium and pelopium (the latter turning out to be a mixture of tantalum and niobium). The confusion persisted until 1866, when the Swiss chemist Jean-Charles Galissard de Marignac definitively separated the two elements by fractional crystallization of complex fluorides.
The isolation of pure tantalum metal was extremely difficult. Early attempts produced impure powders. It was not until 1903 that the German chemist Werner von Bolton succeeded in producing ductile tantalum metal by electrolytic reduction of molten potassium fluorotantalate (K₂TaF₇). This method paved the way for industrial applications of tantalum. The process was improved in the 1920s to allow the production of tantalum wire for incandescent lamps.
Tantalum is present in the Earth's crust at an average concentration of about 1.7 ppm (parts per million), making it rarer than uranium but more abundant than gold. There are no significant deposits of pure tantalum; it is always associated with other elements in complex minerals. The main ores are:
World production of tantalum is about 1,800 to 2,000 tons per year. The main producers are Rwanda, the Democratic Republic of Congo, Brazil, China, and Ethiopia. Due to its rarity and strategic applications, tantalum is an expensive metal, with typical prices of 200 to 400 dollars per kilogram (or more during supply tensions). Demand is mainly driven by electronics (capacitors) and superalloys.
Tantalum (symbol Ta, atomic number 73) is a transition metal of the 6th period, located in group 5 (formerly VB) of the periodic table, with vanadium and niobium. Its atom has 73 protons, usually 108 neutrons (for the unique stable isotope \(\,^{181}\mathrm{Ta}\)) and 73 electrons with the electronic configuration [Xe] 4f¹⁴ 5d³ 6s². This configuration has three electrons in the 5d subshell, characteristic of group 5 transition metals.
Tantalum is a gray-blue, shiny, very dense (16.4 g/cm³), ductile metal with excellent thermal and electrical conductivity. Its melting point is extremely high (3017 °C), classifying it among refractory metals. Tantalum has a body-centered cubic (BCC) crystal structure at room temperature. It is paramagnetic and has low thermal expansion. Its hardness is moderate but can be increased by mechanical treatments or alloys.
The most remarkable property of tantalum is its exceptional resistance to corrosion. At room temperature, it is practically inert: it does not react with air (thanks to a protective Ta₂O₅ oxide layer), resists most acids (including aqua regia), and is only attacked by hydrofluoric acid, hot concentrated alkaline solutions, and some molten salts. This exceptional chemical inertness is due to the formation of an extremely stable, adherent, and protective Ta₂O₅ oxide layer.
Tantalum melts at 3017 °C (3290 K) - one of the highest melting points among metals - and boils at 5458 °C (5731 K). Its electrical conductivity is good (about 13% that of copper) and its thermal conductivity is moderate. Tantalum retains its mechanical properties at high temperatures, making it a valuable material for high-temperature applications.
Melting point of tantalum: 3290 K (3017 °C) - 3rd highest among metals after tungsten and rhenium.
Boiling point of tantalum: 5731 K (5458 °C).
Density: 16.4 g/cm³ - very dense, comparable to gold.
Crystal structure at room temperature: Body-centered cubic (BCC).
Corrosion resistance: Exceptional, almost inert at room temperature.
| Isotope / Notation | Protons (Z) | Neutrons (N) | Atomic mass (u) | Natural abundance | Half-life / Stability | Decay / Remarks |
|---|---|---|---|---|---|---|
| Tantalum-180 — \(\,^{180}\mathrm{Ta}\,\) | 73 | 107 | 179.947466 u | ≈ 0.012 % | > 1.2×10¹⁵ years | Nuclear metastable isomer (180mTa). Only known natural isomer, extremely rare. |
| Tantalum-181 — \(\,^{181}\mathrm{Ta}\,\) | 73 | 108 | 180.947996 u | ≈ 99.988 % | Stable | Only stable isotope of tantalum, representing almost all natural tantalum. |
| Tantalum-182 — \(\,^{182}\mathrm{Ta}\,\) | 73 | 109 | 181.950152 u | Synthetic | ≈ 114.43 days | Radioactive (β⁻). Naturally extinct isotope, important for Ta-W dating in cosmochemistry. |
N.B.:
Electron shells: How electrons are organized around the nucleus.
Tantalum has 73 electrons distributed over six electron shells. Its electronic configuration [Xe] 4f¹⁴ 5d³ 6s² has a completely filled 4f subshell (14 electrons) and three electrons in the 5d subshell. This configuration can also be written as: K(2) L(8) M(18) N(18) O(32) P(5), or in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d³ 6s².
K shell (n=1): contains 2 electrons in the 1s subshell. This inner shell is complete and very stable.
L shell (n=2): contains 8 electrons distributed as 2s² 2p⁶. This shell is complete, forming a noble gas configuration (neon).
M shell (n=3): contains 18 electrons distributed as 3s² 3p⁶ 3d¹⁰. This complete shell contributes to electronic screening.
N shell (n=4): contains 18 electrons distributed as 4s² 4p⁶ 4d¹⁰. This shell forms a stable structure.
O shell (n=5): contains 32 electrons distributed as 5s² 5p⁶ 4f¹⁴ 5d³. The completely filled 4f subshell and the three 5d electrons give tantalum its transition metal properties.
P shell (n=6): contains 5 electrons in the 6s² and 5d³ subshells.
Tantalum effectively has 5 valence electrons: two 6s² electrons and three 5d³ electrons. Tantalum exhibits several oxidation states, but the most stable and common is +5. In this state, tantalum loses its two 6s electrons and three 5d electrons to form the Ta⁵⁺ ion with the electronic configuration [Xe] 4f¹⁴. This ion has a completely filled 4f subshell and is diamagnetic.
Tantalum can also form compounds in lower oxidation states (+4, +3, +2, +1, 0 and even -1 and -3 in some complexes), but these are less stable and generally reducing. For example, TaCl₅ (tantalum pentachloride) is the most common compound in the +5 state, while TaCl₄ (tantalum tetrachloride) represents the +4 state and is air-sensitive. The chemistry of tantalum is therefore dominated by the +5 state, where it chemically resembles niobium (Nb⁵⁺) but with a slightly smaller ionic size and stronger Lewis acidity.
Tantalum metal is remarkably stable in air at room temperature due to the formation of a protective Ta₂O₅ oxide layer. At high temperatures (above 300 °C), it gradually oxidizes: 4Ta + 5O₂ → 2Ta₂O₅. Tantalum(V) oxide is a white, very stable, chemically inert solid, and has a high dielectric constant (κ ~ 25). It is this oxide that gives tantalum its exceptional corrosion resistance. In fine powder form, tantalum can be pyrophoric.
Tantalum is practically inert to water and water vapor, even at high temperatures. It resists most acids, including concentrated hydrochloric acid, concentrated sulfuric acid (up to 150 °C), concentrated nitric acid, and even aqua regia. It is only significantly attacked by:
This exceptional resistance makes it a material of choice for chemical equipment.
Tantalum reacts with halogens at moderate temperatures to form pentahalides: 2Ta + 5F₂ → 2TaF₅; 2Ta + 5Cl₂ → 2TaCl₅. Tantalum pentachloride (TaCl₅) is a very hygroscopic white solid, used as a precursor in chemical vapor deposition and organic synthesis. Tantalum reacts with nitrogen at high temperature (>300 °C) to form tantalum nitride (TaN), with carbon to form tantalum carbide (TaC, one of the most refractory materials known, melting point ~3880 °C), and with hydrogen to form hydrides (TaH).
The most remarkable property of tantalum, after its corrosion resistance, is its exceptional biocompatibility. Tantalum is totally biocompatible: it is non-toxic, does not cause allergic reactions, and does not interfere with biological processes. Moreover, its oxide Ta₂O₅ is also biocompatible and forms a stable layer that does not dissolve in body fluids. These properties, combined with its mechanical strength and ability to be machined with precision, make tantalum an ideal material for medical implants.
The most important application of tantalum is its use in electronic capacitors. Tantalum capacitors account for about 50% of the world's consumption of this metal. They are essential in virtually all electronic devices: mobile phones, computers, medical equipment, automotive systems, etc. Their popularity comes from their excellent combination of properties: high volumetric capacity, stability, reliability, and wide operating temperature range.
Tantalum capacitors are electrolytic capacitors that use tantalum metal as the anode. The anode is made of sintered tantalum powder (to maximize surface area) or tantalum foil. A thin layer of tantalum oxide (Ta₂O₅) formed by anodization serves as the dielectric. The cathode is usually manganese dioxide (MnO₂) or a conductive polymer. This structure allows for very high capacities in a small volume.
Tantalum capacitors are particularly used in:
Tantalum is one of the most biocompatible materials known. It offers several advantages for medical applications:
An important development is porous tantalum (Trabecular Metal™), which mimics the structure of spongy bone. This material has a porosity of about 75-80%, allowing bone growth inside the implant (osteointegration). Porous tantalum implants are particularly used for revision joint prostheses (replacement of failed implants) where bone fixation is problematic.
Thanks to its exceptional corrosion resistance, tantalum is used to manufacture equipment for handling aggressive chemicals:
Tantalum is often used as a coating on less expensive metals (steel, copper) or in combination with glass (tantalized glass) to reduce costs.
Tantalum is an important alloying element in nickel-, cobalt-, and iron-based superalloys for high-temperature applications. It improves:
These alloys are used in gas turbine blades (aeronautics, power generation), combustion chambers, and space propulsion systems.
Tantalum carbide (TaC) and tungsten carbide alloys containing tantalum are used for high-performance cutting tools. Tantalum improves wear resistance, hot hardness, and deformation resistance of tools. These tools are used for machining steels, cast irons, and superalloys.
Tantalum metal and its insoluble compounds (such as Ta₂O₅ oxide) have very low chemical toxicity. Tantalum is considered biologically inert and non-toxic. Tantalum powders can cause mechanical irritation (like any fine powder), but no specific toxic effects. Soluble tantalum compounds (such as potassium fluorotantalate K₂TaF₇) have moderate toxicity, mainly through irritation.
The exceptional biocompatibility of tantalum is demonstrated by its extensive and safe use in medicine for decades. In-depth studies have shown no carcinogenic, mutagenic, or teratogenic effects. Tantalum implants can remain in the body for the patient's lifetime without causing adverse reactions.
The main environmental and social problem associated with tantalum concerns its extraction, particularly that of coltan (columbite-tantalite) in the Democratic Republic of Congo (DRC) and the African Great Lakes region. The problems include:
In response, initiatives such as the OECD's "Due Diligence Guidance" for supply chains of minerals from conflict areas, and certification programs such as the "Conflict-Free Smelter Program" have been developed to ensure responsible sourcing.
Tantalum is widely recycled, with an estimated recycling rate of 20-30%. Sources of recycling include:
Recycling is economically attractive due to the high price of tantalum and helps reduce pressure on mines. However, the collection and sorting of tantalum-containing waste remains a challenge, especially for small electronic devices.
Occupational exposure to tantalum occurs mainly in mines, processing plants, manufacturers of electronic and medical equipment, and industries using tantalum equipment. Standard precautions for metal dusts apply. No specific occupational exposure limit for tantalum is established in most countries, but general recommendations for heavy metal dusts apply (typically 5-10 mg/m³ for total dust).
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