What Is Chromium?
QUICK ANSWER
Chromium is a chemical element with atomic number 24 and the symbol Cr. It's a hard, lustrous, silvery-white transition metal that doesn't tarnish, which is why chrome plating gives such durable mirror finishes. Chromium is essential in stainless steel and used in pigments, leather tanning, and chrome-plated parts on cars.
Chromium is the element that makes stainless steel actually stainless. The same metal that gives car bumpers their mirror finish protects steel from rust by forming an invisible oxide layer. From plumbing fixtures to surgical instruments to the chrome accents on classic cars, chromium's combination of hardness, shine, and corrosion resistance has made it indispensable for over a century.
Where is chromium on the periodic table?
Chromium has atomic number 24, the symbol Cr, and sits in group 6 of the periodic table among the transition metals, alongside molybdenum and tungsten. Its atomic mass is about 52. Four stable isotopes exist (Cr-50, Cr-52, Cr-53, Cr-54), with Cr-52 being most common at 83.8%. Chromium was discovered in 1797 by French chemist Louis-Nicolas Vauquelin from a Siberian mineral called crocoite. The name comes from the Greek chroma meaning 'color' because chromium compounds come in such a wide range of bright colors.
What are the properties of chromium?
Chromium is one of the hardest metals known, with a Mohs hardness of about 8.5, harder than most steels. The melting point is 1,907°C and density is 7.19 g/cm³. The metal takes an exceptional polish and forms a thin protective oxide layer almost instantly when exposed to air, which prevents further corrosion. This same oxide layer is what makes stainless steel stainless. Pure chromium is brittle, which is why it's almost always alloyed with other metals rather than used alone. Chromium is paramagnetic and has an unusual antiferromagnetic ordering below 38°C.
What is chromium used for?
Stainless steel consumes the vast majority of chromium production. By adding at least 10.5% chromium to iron and carbon, the resulting alloy forms a self-healing oxide layer that prevents rust. Chrome plating, a thin layer of chromium electrodeposited on other metals, provides decorative and corrosion-resistant finishes on car parts, appliances, plumbing fixtures, and tools. Chromium pigments produce bright yellow, green, and orange colors in paints. Leather tanning uses chromium salts to make leather flexible and water-resistant. Superalloys with chromium tolerate extreme heat in jet engines.
Is chromium safe?
Chromium safety depends entirely on its chemical form. Trivalent chromium (Cr³⁺) is the form found in food and is an essential trace nutrient that helps regulate blood sugar, though needs are tiny. Hexavalent chromium (Cr⁶⁺) is a known carcinogen and toxic, infamous as the contaminant in the Erin Brockovich case. Workers in chrome plating, leather tanning, and stainless steel welding can be exposed to hexavalent chromium and have higher cancer risks. The two forms have completely different toxicity profiles even though they're the same element.
Chromium gives stainless steel its rust resistance, chrome bumpers their shine, and your jet engine its heat tolerance. The same element exists as both a harmless nutrient and a deadly carcinogen depending on its chemical form, making chromium one of the more chemically complex elements in everyday use.
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