Although iron readily reacts, high purity iron, called electrolytic iron, has better corrosion resistance. Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. However, iron reacts readily with oxygen and water to give brown to black hydrated iron oxides, commonly known as rust. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. The iron and steel industry is thus very important economically, and iron is the cheapest metal, with a price of a few dollars per kilogram or per pound (see Metal#uses). In the modern world, iron alloys, such as steel, stainless steel, cast iron and special steels, are by far the most common industrial metals, because of their mechanical properties and low cost. That event is considered the transition from the Bronze Age to the Iron Age. Humans started to master that process in Eurasia during the 2nd millennium BCE and the use of iron tools and weapons began to displace copper alloys, in some regions, only around 1200 BCE. Iron ores, by contrast, are among the most abundant in the Earth's crust, although extracting usable metal from them requires kilns or furnaces capable of reaching 1,500 ☌ (2,730 ☏) or higher, about 500 ☌ (932 ☏) higher than that required to smelt copper. In its metallic state, iron is rare in the Earth's crust, limited mainly to deposition by meteorites. It is the fourth most common element in the Earth's crust. It is, by mass, the most common element on Earth, right in front of oxygen (32.1% and 30.1%, respectively), forming much of Earth's outer and inner core. It is a metal that belongs to the first transition series and group 8 of the periodic table. Iron ( / ˈ aɪ ər n/) is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26.
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