Chromium is a chemical element with the symbol Cr and atomic number 24. It is a element of Group 6 (VIb) of the periodic table, a hard, steel-gray metal that takes a high polish and is used in alloys to increase strength and corrosion resistance.
It is also highly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, with almost 90% of infrared light being reflected. The name of the element is derived from the Greek word chroma, meaning color, because many Cr compounds are intensely colored.
Chromium is a relatively abundant element in Earth’s crust, It is not free metal is never found in nature. Most ores consist of the mineral chromite, the ideal formula of which is FeCr2O4. It is widely dispersed in natural deposits, which are usually contaminated with oxygen, magnesium, aluminum, and silica; their chromium content varies from 42 to 56 percent. One of the chief uses of Cr is in ferrous alloys, for which the pure metal is not required. Accordingly, chromite is often reduced with carbon in a furnace, producing the alloy ferrochromium, which contains iron and chromium in an atom ratio of approximately 1 to 2.
To obtain pure Cr, chromite is first treated with molten alkali and oxygen, converting all of the chromium to the alkali chromate, and the latter is dissolved in water and eventually precipitated as sodium dichromate, Na2Cr2O7. The dichromate is then reduced with carbon to chromium sesquioxide, Cr2O3, and that oxide in turn is reduced with aluminum to give the chromium metal.
Physical Properties Chromium
- It is extremely hard, and is the third hardest element behind carbon (diamond) and boron. Its Mohs hardness is 8.5, which means that it can scratch samples of quartz and topaz, but can be scratched by corundum.
- It is highly resistant to tarnishing, which makes it useful as a metal that preserves its outermost layer from corroding, unlike other metals such as copper, magnesium, and aluminium.
- Chromium has a melting point of 1907 °C, which is relatively low compared to the majority of transition metals. However, it still has the second highest melting point out of all the Period 4 elements, being topped by vanadium by 3 °C at 1910 °C.
- It has unique magnetic properties in the sense that chromium is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, its magnetic ordering changes to paramagnetic.