- Chromium (Cr) is a hard, lustrous, steel-gray transition metal with atomic number 24, positioned in Group 6 of the periodic table.
- It has six valence electrons—one in the 4s subshell and five in the 3d subshell—and exhibits oxidation states ranging from −2 to +6, with +3 and +6 being the most common in stable compounds. Its atomic structure consists of twenty-four protons, typically twenty-eight neutrons, and twenty-four electrons arranged in four shells.
- Naturally occurring chromium is composed of four stable isotopes: chromium-52 (⁵²Cr), the most abundant at about 83.79%; chromium-53 (⁵³Cr) at 9.50%; chromium-50 (⁵⁰Cr) at 4.35%; and chromium-54 (⁵⁴Cr) at 2.36%.
- Chromium occurs in nature primarily as the mineral chromite (FeCr₂O₄), the only commercially significant source. Major deposits are found in South Africa, Kazakhstan, India, and Turkey. Chromium is never found in its pure metallic state in nature, due to its high reactivity at elevated temperatures, but its compounds are widespread in rocks, soils, and living organisms.
- The element was discovered in 1797 by French chemist Louis Nicolas Vauquelin, who isolated it from crocoite (PbCrO₄), a bright orange mineral. The following year, Vauquelin successfully obtained metallic chromium by heating chromium oxide in a charcoal furnace. The name “chromium” comes from the Greek chroma, meaning “color,” due to the wide variety of colors displayed by its compounds.
- Chromium’s most famous property is its exceptional corrosion resistance, which is due to the formation of a stable, protective oxide layer on its surface. This makes it an essential component in stainless steel, where it typically constitutes 10–20% of the alloy. Chromium-plating (chrome plating) is widely used for decorative and protective finishes on automotive parts, household fixtures, and tools, providing both shine and resistance to tarnish.
- In industry, chromium compounds have many applications. Chromium(VI) oxide (CrO₃) and other hexavalent chromium compounds are used in electroplating, pigments, and wood preservation, although their toxicity has led to strict regulations. Chromium(III) oxide (Cr₂O₃) is a stable green pigment used in paints, ceramics, and glassmaking. Chromium salts are also used in tanning leather, imparting durability and resistance to decay.
- Chemically, chromium shows diverse reactivity depending on its oxidation state. In the +3 state, it forms stable, often green compounds; in the +6 state, it forms strong oxidizing agents, such as chromates and dichromates, which are typically yellow to orange. Chromium metal is resistant to oxidation and tarnishing, even at high temperatures, which makes it valuable for high-temperature alloys and refractory materials.
- Biologically, chromium in trace amounts (as trivalent chromium, Cr³⁺) is considered an essential nutrient for humans, contributing to the regulation of blood sugar through its role in insulin function. However, hexavalent chromium (Cr⁶⁺) is highly toxic and carcinogenic, capable of causing severe health problems through inhalation or ingestion.
- Environmentally, chromium mining and industrial use can lead to contamination of soil and water, particularly with Cr⁶⁺ compounds, which are highly mobile and toxic. Modern environmental controls and waste-treatment processes aim to reduce chromium pollution and recover chromium from industrial waste streams.
