Tungsten metal properties
Tungsten (W) is a chemical element or strong, silvery transition metal of Group 6 (VIB) of the periodic table, used for making steel and lamp filaments due to its hardness. The symbol of tungsten W is derived from the name Wolfram which is still used in some literature. The properties of tungsten and molybdenum are very similar but the chemistry of these two metals are slightly different. Tungsten has the highest melting point among all metals and second after carbon among all periodic table elements.
The atomic number of tungsten is 74 and electronic configuration [Xe] 4f14 5d4 6s2. It is the last metal in the third transition series in which all the d-electrons participate metallic bonding. The metal is very hard, high melting, and relatively unreactive due to the protective oxide layer. It forms typically metallic body-centered cubic crystal lattice like molybdenum. The metal chromium, molybdenum, and tungsten are the members of Group-VI in the periodic table. Therefore, the common oxidation number or state of tungsten is +6 (VI).
|Properties of Tungsten|
|Electronic configuration||[Xe] 4f14 5d4 6s2|
|Melting point||3695 K (3422 °C, 6192 °F)|
|Boiling point||6203 K (5930 °C, 10706 °F)|
|Molar heat capacity||24.27 J mol-1K-1|
|Electrical resistivity||52.80 nΩ·m|
|Atomic radius||139 pm|
|Covalent radius||162±7 pm|
|Electronegativity||Pauling scale – 2.36|
|Ionization energy||1st – 770 kJ/mol
2nd – 1700 kJ/mol
History and Occurrence
The mineral tungsten (Swedish heavy stone CaWO4) was studied in 1751 by Cronstedt. In 1981 Carl Wilhelm Scheele discovered new oxide from this mineral. Two years later Spanish chemists José and Fausto Elhuyar (two brothers) separated the same oxide from the mineral known as wolframite. They isolated the metal tungsten by carbon reduction process by charcoal. The symbol of the metal W is derived from the name Wolfram.
Tungsten occurs in nature mostly as wolframite, Fe(Mn)WO4, and scheelite or calcium tungstate, CaWO4. The main reserve place of the metal in China, the United States, South Korea, Bolivia, Portugal, and Russia.
The metal tungsten has four natural isotopes (182W, 183W, 184W, and 186W) and one radioactive isotope (180W). The radioactive isotope of the metal has very long lives. All the five isotopes radioactive decay to form hafnium by nuclear reaction. Another 30 radioactive isotopes of metal are observed. These can be obtained by the different types of nuclear reactions.
Production of Tungsten Metal
The concentrated ore of the metal is roasted in the air with sodium carbonate or fused sodium hydroxide (NaOH). The sodium tungstic acid is extracted with water and treated with water. The precipitated tungstic acid is heated to obtained WO3. Pure tungsten metal is obtained by the reduction of metal oxide with zinc, aluminum, or hydrogen.
The metals Mo and W have very high melting points and they are obtained initially in the form of powder. It is converted to a massive state by compression under hydrogen at high temperatures.
Chemistry and Chemical Compounds
The chemistry of tungsten is close to molybdenum. The most common and stable oxidation state of the metal is +6 but the +5 and +4 states are also stable. Strong reducing properties of the metal are not met before +3 and +2 states. The oxides, fluorides, oxoacids of tungsten formed in different oxidation states.
The yellow WO3 is the end product of heating other compounds of tungsten in the air. It is insoluble in acids. WO3, 2H2O is formed in strongly acidic media. It is called tungstic acid but it not the hydrated acids. A few lower oxides like WO2 were obtained by the reduction of trioxide with hydrogen. The acidic nature of the oxides decreases with decreasing oxidation state.
Halides and oxohalides
The hexahalides of the metal are obtained by the direct combination. The colorless fluoride like WF6 is more stable. It is quite reactive and readily reacts with oxygen and moisture to form oxohalide and HF. WBr6 is unstable, decompose on gentle worming. The oxohalides of the metal are two types
- WOX4 (X = F, Cl, Br): They are volatile, solid covalent compounds, hydrolyzed to give WO3.
- WO2X2 (X = F, Cl, Br, I): The yellow colour WO2Cl2 decomposes above 200 °C to form WO3 and red WOCl4.
The petahalides of tungsten metal are known. The yellow halide like WF5 is formed by the reduction of WF6 at 500 to 800 °C. Above 30 °C the yellow solid disproportionate reaction between WF4 and WF6. The tetrafluoride of the metal is formed by the reduction of hexafluoride with benzene or other hydrocarbons. It is a nonvolatile red-brown chemical compound.
Lower oxidation state of tungsten
The lower oxidation state of tungsten is unstable and strongly reducing in nature. The metal only forms WCl3 and WBr3 in +3 oxidation state. W(II) is completely unstable.
The complex halide of W(II) is W6X12 type (where X = chlorine, bromine, iodine). It forms cluster compound like (W6X8)+4 X4–. The oxidation state zero is observed in the carbonyl complexes like W(CO)6.
Analytical reactions of tungsten
Hydrochloric acid addition to a tungstate, hydrated tungstic acid is precipitated. On heating, the white precipice of tungstic acid turns yellow. It may be confirmed by heating the precipitate with Zn and HCl or SnCl2 when a blue precipitate is formed. Tungstates are precipitated as benzidine tungstate from a dilute 0.1N sulfuric acid. The precipitate is ignited to WO3 and weighed. At pH scale of 7 to 8 tungstates can be precipitated as BaWO4.
Uses of Tungsten Metal
- The pure tungsten metal is used in making filaments of old-style electric lamps or bulbs and anticathodes in x-ray tubes.
- Tungsten is an important alloying metal for high-speed tools due to its hardness. It forms several very hard alloys such as satellite (cobalt, chromium, and tungsten) and Widia metal (WC + 10 percent copper).
- The compound tungsten(IV) sulfide used in high-temperature lubricant and calcium/magnesium tungstates are used widely in fluorescent lighting. The oxide form of tungsten is used as a chemical catalyst in power plants. It converts nitrogen oxide to nitrogen and water in presence of ammonia.
- Due to hardness tungsten carbide (WC) is an important metal carbide for mining and petroleum industries. It is used for making different types of high-speed cutting tools. It can be prepared by mixing tungsten powder and carbon powder at 2200 °C.
Tungsten in biological systems
Tungsten is the only element in the third transition series which function on the biological system. Protein containing the metal is rare and is mainly found in thermophilic anaerobes. It also occurs in several enzymes with molybdenum. The mode of the biological function of transition metal tungsten centre in enzymes is involved oxo-transfer of W(VI) to the substrate.