TUNGSTEN: HISTORY, PROPERTIES, STRUCTURE, USES - CHEMISTRY - 2024

The tungsten, tungsten or tungsten heavy metal is a transition whose chemical symbol is W. Located in the period 6 group 6 of the periodic table, and atomic number 74. Its name has two etymological meanings: hard stone and wolf foam; The second is because this metal is also known as tungsten.

It is a silver-gray metal and, although it is brittle, it has great hardness, density, and high melting and boiling points. Therefore, it has been used in all those applications that involve high temperatures, pressures or mechanical forces, such as drills, projectiles, or radiation-emitting filaments.

Tungsten bar with its partially oxidized surface. Source: Hi-Res Images of Chemical Elements

The best known use for this metal, at a cultural and popular level, is in the filaments of electric bulbs. Whoever has handled them will realize how fragile they are; however, they are not made of pure tungsten, which is malleable and ductile. In addition, in metallic matrices such as alloys, it provides excellent resistance and hardness.

It is characterized and distinguished by being the metal with the highest melting point, and also being denser than lead itself, surpassed only by other metals such as osmium and iridium. Likewise, it is the heaviest metal known to fulfill some biological role in the body.

The tungstate anion, WO ₄ ^2-, participates in most of its ionic compounds, which can polymerize to form clusters in an acid medium. On the other hand, tungsten can form intermetallic compounds, or be sintered with metals or inorganic salts so that its solids acquire different shapes or consistencies.

It is not very abundant in the earth's crust, with only 1.5 grams of this metal per ton. Furthermore, as it is a heavy element, its origin is intergalactic; specifically from supernova explosions, which must have thrown “jets” of tungsten atoms towards our planet during its formation.

History

Etymology

The history of tungsten or wolfram has two faces just like their names: one Swiss, and the other German. In the 1600s, in regions currently occupied by Germany and Austria, miners worked to extract copper and tin to produce bronzes.

By then the miners found themselves with a thorn in the process: there was an extremely difficult mineral to melt; mineral consisting of wolframite, (Fe, Mn, Mg) WO ₄, which retained or "devoured" the tin as if it were a wolf.

Hence the etymology for this element, 'wolf' for wolf in Spanish, a wolf that ate tin; and 'ram' of foam or cream, whose crystals resembled a long black fur. It was thus that the name 'wolfram' or 'wolfram' arose in honor of these first observations.

In 1758, on the Swiss side, a similar mineral, scheelite, CaWO ₄, was named 'tung sten', which means 'hard stone'.

Both names, wolfram and tungsten, are widely used interchangeably, depending solely on the culture. In Spain, for example, and in western Europe, this metal is best known as tungsten; while in the American continent the name tungsten predominates.

Recognition and discovery

It was known then that between the seventeenth and eighteenth centuries there were two minerals: wolframite and scheelite. But, who saw that there was a metal in them different from the others? They could only be characterized as minerals, and it was in 1779 that the Irish chemist Peter Woulfe carefully analyzed tungsten and deduced the existence of tungsten.

On the Swiss side, again, Carl Wilhelm Scheele in 1781 was able to isolate tungsten as WO ₃; and even more, he obtained tungstic (or tungstic) acid, H ₂ WO ₄ and other compounds.

However, this was not enough to get to the pure metal, since it was necessary to reduce this acid; that is, subjecting it to a process such that it detaches from oxygen and crystallizes as metal. Carl Wilhelm Scheele did not have the appropriate furnaces or methodology for this chemical reduction reaction.

It is here where the Spanish brothers d'Elhuyar, Fausto and Juan José, came into action, who reduced both minerals (wolframite and scheelite) with coal, in the city of Bergara. The two of them are awarded the merit and honor of being the discoverers of metallic tungsten (W).

Steels and bulbs

Any light bulb with a tungsten filament. Source: Pxhere.

Like other metals, its uses define its history. Among the most prominent at the end of the 19th century, were the steel-tungsten alloys, and the tungsten filaments to replace the carbon ones inside the electric light bulbs. It can be said that the first light bulbs, as we know them, were marketed in 1903-1904.

Properties

Physical appearance

It is a lustrous silver-gray metal. Fragile but very hard (not to be confused with toughness). If the piece is of high purity it becomes malleable and hard, as much or more as several steels.

Atomic number

74.

Molar mass

183.85 g / mol.

Melting point

3422 ° C.

Boiling point

5930 ° C.

Density

19.3 g / mL.

Heat of fusion

52.31 kJ / mol.

Heat of vaporization

774 kJ / mol.

Molar heat capacity

24.27 kJ / mol.

Moh's hardness

7.5.

Electronegativity

2.36 on the Pauling scale.

Atomic radio

139 pm

Electrical resistivity

52.8 nΩ · m at 20 ° C.

Isotopes

It occurs predominantly in nature as five isotopes: ¹⁸² W, ¹⁸³ W, ¹⁸⁴ W, ¹⁸⁶ W and ¹⁸⁰ W. According to the molar mass of 183 g / mol, which averages the atomic masses of these isotopes (and the others thirty radioisotopes), each tungsten or tungsten atom has about one hundred and ten neutrons (74 + 110 = 184).

Chemistry

It is a metal highly resistant to corrosion, as its thin layer of WO ₃ protects it against the attack of oxygen, acid and alkalis. Once dissolved and precipitated with other reagents, its salts are obtained, which are called tungstates or wolframates; tungsten usually has an oxidation state of +6 in them (assuming there are W ⁶⁺ cations).

Acid clustering

Decatungstate, an example of tungsten polyoxometalates. Source: Scifanz

Chemically tungsten is quite unique because its ions tend to clustering to form heteropoly acids or polyoxometalates. What are they? They are groups or clusters of atoms that come together to define a three-dimensional body; Mainly, one with a spherical cage-like structure, in which they “enclose” another atom.

It all starts from the tungstate anion, WO ₄ ^2-, which rapidly protonates in an acid medium (HWO ₄ ^-) and binds with a neighboring anion to form ^2-; and this in turn joins with another ^2- to originate the ^4-. So on until there are several politungstats in solution.

Paratungstates A and B, ^6- and H ₂ W ₁₂ O ₄₂ ^10-, respectively, are one of the most prominent of these polyanions.

It can be challenging to come up with your Lewis sketch and structures; but in principle it is enough to visualize them as sets of WO ₆ octahedra (upper image).

Note that these grayish octahedra end up defining the decatungstate, a politungstat; If a heteroatom (for example, phosphorus) contained within it, it would then be a polyoxometalate.

Structure and electronic configuration

Crystalline phases

Tungsten atoms define a crystal with a body-centered cubic (bcc) structure. This crystalline form is known as the α phase; while the β phase is also cubic, but a little more dense. Both phases or crystalline forms, α and β, can coexist in equilibrium under normal conditions.

The crystalline grains of the α phase are isometric, whereas those of the β phase resemble columns. Regardless of the crystal, it is governed by the metallic bonds that hold the W atoms tightly together. Otherwise, the high melting and boiling points, or the high hardness and density of tungsten, could not be explained.

Metallic bond

Tungsten atoms have to be tightly bound somehow. To make a conjecture, the electron configuration of this metal must first be observed:

4f ¹⁴ 5d ⁴ 6s ²

The 5d orbitals are very large and fuzzy, which would imply that between two nearby W atoms there are effective orbital overlaps. Also, the 6s orbitals contribute to the resulting bands, but to a lesser degree. While the 4f orbitals are "deep in the background" and therefore their contribution to the metallic bond is less.

This, the size of the atoms, and the crystalline grains, are the variables that determine the hardness of tungsten and its density.

Oxidation states

In metallic tungsten or wolfram, the W atoms have zero oxidation state (W ⁰). Returning to the electronic configuration, the 5d and 6s orbitals can be "emptied" of electrons depending on whether W is in the company of highly electronegative atoms, such as oxygen or fluorine.

When the two 6s electrons are lost, tungsten has a +2 oxidation state (W ²⁺), which causes its atom to contract.

If it also loses all the electrons in its 5d orbitals, its oxidation state will become +6 (W ⁶⁺); From here it cannot become more positive (in theory), since the 4f orbitals, being internal, would require great energies to remove their electrons. In other words, the most positive oxidation state is +6, where the tungsten is even smaller.

This tungsten (VI) is very stable in acidic conditions or in many oxygenated or halogenated compounds. Other possible and positive oxidation states are: +1, +2, +3, +4, +5 and +6.

Tungsten can also gain electrons if it combines with atoms less electronegative than itself. In this case, its atoms get bigger. It can gain a maximum of four electrons; that is to say, have an oxidation state of -4 (W ^4-).

Obtaining

It was mentioned earlier that tungsten is found in the minerals wolframite and scheelite. Depending on the process, two compounds are obtained from them: tungsten oxide, WO ₃, or ammonium paratungstate, (NH ₄) ₁₀ (H ₂ W ₁₂ O ₄₂) · 4H ₂ O (or ATP). Either of them can be reduced to metallic W with carbon above 1050 ° C.

It is not economically profitable to produce tungsten ingots, as they would need a lot of heat (and money) to melt them. That is why it is preferred to produce it in powder form to treat it at once with other metals to obtain alloys.

It is worth mentioning that China is the country with the largest tungsten production worldwide. And in the American continent, Canada, Bolivia and Brazil also occupy the list of the largest producers of this metal.

Applications

A ring made of tungsten carbide - an example of how the hardness of this metal can be used to immortalize and harden materials. Source: SolitaryAngel (SolitaryAngel)

Here are some of the known uses for this metal:

-Its salts were used to color cottons from old theaters attire.

-Combined with steel it hardens it even more, being able even to resist mechanical cuts at high speeds.

-Sintered tungsten filaments have been used for over a hundred years in electric bulbs and halogen lamps. Also, due to its high melting point it has served as a material for cathode ray tubes, and for the nozzles of rocket engines.

-Replaces lead in the manufacture of projectiles and radioactive shields.

-Tungsten nanowires can be used in pH and gas sensitive nanodevices.

-Tungsten catalysts were used to deal with sulfur production in the oil industry.

-Tungsten carbide is the most widely used of all its compounds. From the strengthening of cutting and drilling tools, or the manufacture of pieces of military armaments, to machining wood, plastics and ceramics.

Risks and precautions

Biological

Being a relatively rare metal in the earth's crust, its negative effects are scarce. In acidic soils, polyungstates may not affect the enzymes that use molybdate anions; but in basic soils, WO ₄ ^2- does intervene (positively or negatively) in the metabolic processes of MoO ₄ ^2- and copper.

Plants, for example, can absorb soluble tungsten compounds, and when an animal eats them and then after consuming its meat, W atoms enter our bodies. Most are expelled in the urine and feces, and little is known what happens to the rest of them.

Animal studies have shown that when they inhale high concentrations of powdered tungsten they develop symptoms similar to those of lung cancer.

By ingestion, an adult human would need to drink thousands of gallons of water enriched with tungsten salts to show appreciable inhibition of the enzymes cholinesterase and phosphatase.

Physical

In general terms, tungsten is a low toxic element, and therefore there are few environmental risks of damage to health.

Regarding metallic tungsten, avoid breathing its dust; and if the sample is solid, it must be borne in mind that it is very dense and that it can cause physical damage if it is dropped or hit other surfaces.

References

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