Tungsten

Tungsten (chemical symbol, W) is a hard refractory metallic element with remarkable resilience which forms the basis for its industrial uses. It is the metal of choice in the filaments and targets of x-ray tubes. There is no evidence that tungsten is required by the human body, although some micro-organisms do use it.

Chemistry

Basic chemistry

Tungsten has the atomic number 74 and a relative atomic weight of 183.84 ¹. It is located in group VIb of the periodic table beneath chromium and molybdenum. It has the highest melting point of any metal at 3422°C, the lowest vapour pressure (low tendency to evaporate) and the highest tensile strength at high temperatures (>1650°C). Its density is 19.3 g/cm³, only very slightly less than gold (19.32 g/cm³) ^1,4. Tungsten has high resistance to electricity and thus is able to produce large amounts of heat when an electrical current passes through it. Tungsten resistivity also increases with an increase in temperature ⁹.

At room temperature, tungsten is a solid steel-gray-to-white transition metal that is hard and brittle. It only becomes ductile (able to be drawn into wires) at a temperature of 700 °C. Thermomechanical processing and alloying with another metal rhenium can improve the ductility of tungsten at lower temperatures ¹⁰.

Radiochemistry

There are four stable isotopes of tungsten, the commonest on earth being W-134 which accounts for 30.6% of the tungsten found terrestrially. Isotopes W-186, W-182 and W-183 account for 28.4%, 26.5% and 14.3% respectively ². A fifth isotope, W-180, is technically unstable but has a half-life ~1.5 × 10¹⁸ years. It accounts for a mere 0.1% terrestrial tungsten ².

At least 38 unstable isotopes have been characterised ².

Medical importance

Tungsten has important niche roles in x-ray tubes and in radiation shielding.

• tungsten is used as the metal in the filament (or cathode) in x-ray tubes due to its very high melting point ⁴

• due to properties of high melting point, high atomic number, and low vapour pressure, tungsten is also chosen as the target material (anode) in x-ray tubes ⁵. A tungsten-rhenium alloy (90%:10% mix) forms the basis for the target in modern x-ray tubes, rhenium improves the durability of the target ⁴

• tungsten is used as a shielding material for FDG by the radiopharmaceutical industry, although it has a lower atomic number than lead it mitigates this with a higher density ⁵

• tungsten is used for some embolisation coils

• tungsten carbide, an extremely hard-wearing material, is used for neurosurgical and dental burrs and drills

Toxicity

Elemental tungsten is relatively inert and toxicology studies have struggled to identify any significant toxicity from it ^3,4.

There is contentious evidence that chronic occupational exposure to tungsten compounds (e.g. tungsten carbide) may have deleterious biological effects in people. Some metal workers have presented with symptoms following long-term cutaneous and/or respiratory exposure to mineral dusts. However, it is very difficult, if not impossible, to separate this out from coexisting dusts of other metals, e.g. cobalt, which are more likely the main culprit in these scenarios ^3,4.

Acute tungsten poisoning has been reported but is very rare ⁴.

History and etymology

In 1758, the Swedish chemist Axel Fredrik Cronstedt found an especially weighty mineral, which he called “tung sten”, literally ‘heavy stone’, and he thought contained a novel element ⁴. Another Swedish chemist, Carl Wilhelm Scheele (1742-1786), isolated tungsten trioxide (WO₃) from an acid derivative of the scheelite (the modern term for the tungsten mineral) in 1781 ^7,8. It was only two years later in 1783 that two Spanish brothers, Juan and Fausto Elhuyar, fully purified the element from wolframite, an ore, by reducing tungsten trioxide with charcoal, and they are recognised as the true ‘discoverers’ of the element ^4,8.

So, they named their new element wolfram, but the name tungsten had already become well-known in the English-speaking chemistry world. In the past, IUPAC had recognised both names, but in 2005 it decided that tungsten would be the official international name for the element ¹.