Chromium
Chromium, symbol Cr, gray metallic element that can take on a high polish. The atomic number of chromium is 24; the element is one of the transition elements of the periodic table (see Periodic Law ). PROPERTIES AND OCCURRENCE The element was discovered in 1797 by the French chemist Louis Nicolas Vauquelin, who named it chromium (Greek chroma, “color”) because of the many different colors of its compounds. Chromium is a common element; overall it ranks about 21st in natural abundance among the elements in crustal rocks. Chromium has an atomic weight of 51.996; the element melts at about 1857° C (about 3375° F), boils at about 2672° C (about 4842° F), and has a specific gravity of 7.2. Chromium can replace part of the aluminum or iron in many minerals, imparting to them their unique colors. Many precious stones owe their color to the presence of chromium compounds (see Gemstones ). Workable ores are rare, however, chromite being the only important one. In chromites and chromic salts, c