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Physical Characteristics
Discovered in 1780 and first isolated in 1781, elemental chromium is a hard and dense gray rock with a lustrous reflective surface. The most common use of chromium is to plate stainless steel and other metals. Chromium can be plated very thin and polished to a mirror finish that is both attractive and resistant to corrosion. Chromium does not exist as a free element in nature. It must be extracted from ore. The most common is chromite, a rock composed of chromium, iron and oxygen.
Chemical Characteristics
Chromium is number 24 on the periodic table of elements with a mass of 51.996. It is a solid at room temperature of 298 kelvins and melts at 2,180 kelvins. On the periodic table, chromium is a transition element in group 6, meaning it has an incomplete d-subshell. Elemental chromium is very hard, rating an 8.5 on the Mohs scale.
Uses and Environmental Considerations
Chromium's most common use is to harden and coat steel with a corrosion-resistant mirror finish. It is also a component in many paints and pigments -- usually green, yellow, red and orange. Humans require trace amounts of chromium in their diets. However, some chromium compounds, such as hexavalent chromium, are toxic and carcinogenic.
Trivia
Trace amounts of chromium is what gives emeralds and rubies their distinctive color. In both stones, chromium atoms replace aluminum ions in the crystal lattice. Crocoite, commonly called Siberian red lead, is a naturally occurring chromium ore that is highly valued as a red pigment in oil paints. The name chromium is from the Greek word for color, "chroma," because the element can form so many different colored compounds. The movie "Erin Brockovich" was based on hexavalent chromium poisoning in ground water.