Chemical Reactivity of Tungsten
Tungsten, designated by the chemical symbol W and atomic number 74, is a transition metal. Transition metals are broadly characterized by their varied oxidation states and typically lower reactivity compared to elements in groups 1 and 2 of the periodic table. Tungsten’s electron configuration and strong metallic bonding contribute to its high melting point and significant stability.
Reaction with Water
Tungsten exhibits exceptional resistance to reaction with water. The bulk metal does not react with water or steam, even when exposed to high temperatures. This inertness under aqueous conditions is a key property that makes tungsten valuable in environments where corrosion by water or moisture would degrade other materials. Only under extreme conditions, such as finely powdered tungsten reacting with superheated steam, can a reaction occur, forming tungsten oxides and hydrogen gas.
Reaction with Air (Oxygen)
At room temperature, tungsten is highly resistant to oxidation in air. Its surface forms a thin, protective layer of oxide that prevents further corrosion, a phenomenon known as passivation. This protective layer ensures the metal’s durability in atmospheric conditions. However, when heated to sufficiently high temperatures (generally above 400 °C) in the presence of air or pure oxygen, tungsten begins to oxidize. The primary product of this reaction is tungsten trioxide ($\text{WO}_3$). This property is exploited in various industrial applications where high-temperature performance in the presence of oxygen is required, though in some cases, such as incandescent light bulb filaments, tungsten is operated in an inert gas atmosphere or vacuum to prevent rapid oxidation at its extremely high operating temperatures.
Toxicity, Radioactivity, and Flammability
- Toxicity: Bulk tungsten metal is generally considered to have low toxicity. It is not known to pose significant health risks under normal handling and exposure conditions. Studies have indicated that insoluble tungsten compounds also show low toxicity. However, some soluble tungsten compounds can exhibit toxicity if ingested or inhaled in large quantities, with the degree of toxicity dependent on the specific compound and its bioavailability.
- Radioactivity: Naturally occurring tungsten is not radioactive. Its most abundant isotope, tungsten-184 ($^{184}$W), is stable. While various artificial radioactive isotopes of tungsten have been produced in laboratory settings, they are not naturally occurring and are not typically found outside specialized research environments.
- Flammability: Tungsten metal in its solid, bulk form is not flammable. Its exceptionally high melting point of 3422 °C means it does not easily ignite or sustain combustion. Nevertheless, like many metals, finely divided tungsten powder can be pyrophoric, meaning it can spontaneously ignite in air, particularly when dispersed as a dust cloud or at elevated temperatures, due to its large surface area.
Illustrative Chemical Reaction
A prominent chemical reaction involving tungsten is its high-temperature oxidation in an oxygen-rich environment. This reaction can be represented by the following equation:
$2W(s) + 3O_2(g) \xrightarrow{\text{heat}} 2WO_3(s)$
This process is fundamental in the industrial preparation of tungsten compounds. For example, in regions with significant tungsten mining, such as parts of China, tungsten ores are often processed to yield tungsten trioxide. This oxide then serves as an intermediate in the production of pure tungsten metal through subsequent reduction steps, commonly using hydrogen gas at high temperatures. Tungsten trioxide itself has applications as a pigment in ceramics and paints, and in the manufacture of electrochromic windows.