|
|
Atomic Number: |
24 |
Melting Point: | 1907 ºC | |
| Atomic Symbol: | Cr | Boiling Point: | 2671 ºC | ||
| Atomic Weight: | 51.996 amu | Density: | 7140
kg/m 3 |
||
| Atomic Radius: |
124.9 pm |
Oxidation States: | 6, 3, 2 | ||
| Covalent Radius: | 127 pm | Electron Configuration: | [Ar]4s13d5 | ||
| van der Waals Radius: |
-- |
State of Matter: | solid |
(Gr. chroma: color) Discovered in 1797 by Vauquelin, who prepared the metal in the following year. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gems, such as ruby, or emerald.
Chromium is a steel-gray, lustrous, hard metal that takes a high polish, is fusible with difficulty, and is resistant to corrosion and tarnishing. The most common oxidation states of chromium are +2, +3, and +6, with +3 being the most stable. +4 and +5 are relatively rare. Chromium compounds of oxidation state 6 are powerful oxidants.
The principal ore is chromite, which is found in Zimbabwe, Russia, New Zealand, Turkey, Iran, Albania, Finland, Democratic Republic of Madagascar, and the Phillippines. The metal is usually produced by reducing the oxide with aluminum.
Chromium is used to harden steel, manufacture stainless steel, and form many useful alloys. It is mostly used in plating to produce a hard, beautiful surface and to prevent corrosion. Chromium gives glass an emerald green color and is widely used as a catalyst.
The refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stability of crystalline structure.
All compounds of chromium are colored. The most important chromates are those of sodium and potassium, the dichromates, and the potassium and ammonium chrome alums. The dichromates are used as oxidizing agents in quantitative analysis, also in tanning leather.
Other compounds are of industrial value; lead chromate is chrome yellow, a valued pigment. Chromium compounds are used in the textile industry as mordants, and by the aircraft and other industries for anodizing aluminum.
Naturally occurring chromium is composed of 3 stable isotopes; 52-Cr, 53-Cr, and 54-Cr. 52-Cr is the most abundant with 83.789% natural abundance. 19 radioisotopes have been characterized with the most stable being 50-Cr with a half-life of (more than) 1.8E17 years, and 51-Cr with a half-life of 27.7025 days. All of the remaining radioactive isotopes have half-lifes that are less than 24 hours and the majority of these have half lifes that are less than 1 minute. This element also has 2 meta states.
Chromium metal and chromium(III) compounds are not usually considered health hazards, but chromium (VI) compounds can be toxic if orally ingested. The lethal dose of poisonous chromium (VI) compounds is about one half teaspoon of material. Most chromium (VI) compounds are irritating to eyes, skin and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury, unless properly treated. Chromium(VI) is an established human carcinogen.
As chromium compounds were/are used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites, now needing environmental cleanup and remediation.
