Nickel was first isolated and classified as an element in 1751 by Axel Fredrik Cronstedt, who initially mistook the ore for a copper mineral, in the cobalt mines of Los, Hälsingland, Sweden. Use of nickel (as natural meteoric nickel–iron alloy) has been traced as far back as 3500 BCE. An iron–nickel mixture is thought to compose Earth's outer and inner cores.
Meteoric nickel is found in combination with iron, a reflection of the origin of those elements as major end products of supernova nucleosynthesis. Even so, pure native nickel is found in Earth's crust only in tiny amounts, usually in ultramafic rocks, and in the interiors of larger nickel–iron meteorites that were not exposed to oxygen when outside Earth's atmosphere.
Pure nickel is chemically reactive but large pieces are slow to react with air under standard conditions because a passivation layer of nickel oxide forms on the surface that prevents further corrosion. Nickel is a hard and ductile transition metal. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a chemical element with symbol Ni and atomic number 28.
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