Nickel

At atomic number 28 with the electron configuration [Ar] 3d8 4s2, nickel is the last of the classic ferromagnetic elements in the first transition series, joining iron and cobalt in its ability to sustain a permanent magnetic moment. Its density of 8.912 g/cm³, electronegativity of 1.91, and first ionization energy of 7.64 eV mark nickel as a robust, moderately reactive metal that nonetheless forms a thin, adherent oxide layer that gives it practical corrosion resistance in air and mild acids. The +2 oxidation state dominates nickel chemistry, producing a characteristic green color in aqueous solutions of nickel(II) salts and in the octahedral complex [Ni(H2O)6]2+. States of +3 and +4 appear in solid-state and organometallic contexts, and nickel(0) complexes play pivotal roles in homogeneous catalysis. Its position at the end of the d-block row means nickel has a nearly full d subshell, influencing both its reactivity and its magnetic behavior.

Nickel's dominant application is in stainless steel, where additions of 8 to 10 percent nickel stabilize the austenite phase, improving corrosion resistance, toughness at low temperatures, and weldability. Roughly two-thirds of all nickel produced goes into stainless steel for construction, food processing, chemical plant equipment, and medical devices. Superalloys — nickel-based high-temperature alloys such as Inconel and Waspaloy — are the second major application, prized for retaining strength and oxidation resistance in jet turbine blades, rocket nozzles, and industrial furnace components operating above 1000 degrees Celsius. Nickel is used to electroplate steel and brass, providing a hard, bright, corrosion-resistant surface on automotive trim, plumbing fixtures, and electronic contacts. Nickel-metal hydride batteries power hybrid vehicles and portable tools, and nickel cobalt manganese oxide cathodes are increasingly common in electric vehicle batteries. In catalysis, Raney nickel — a porous nickel-aluminum alloy — is a workhorse heterogeneous catalyst for hydrogenation of vegetable oils to produce margarine and shortening, and for a range of industrial organic syntheses.

Swedish mineralogist Axel Fredrik Cronstedt isolated nickel in 1751 from an ore called kupfernickel, a name meaning 'devil's copper' or 'Old Nick's copper' in German mining slang — miners were frustrated that the reddish ore resembled copper-bearing rock but yielded no copper when smelted. The element was named nickel after that ore. Earlier in the century, chemists had suspected that kupfernickel contained an unknown metal, and Cronstedt's systematic work conclusively separated the element by reducing its oxide with charcoal. Over the following decades, the full chemical character of nickel was worked out by Torbern Bergman and others who established its similarity to cobalt and iron. In 1889, James Riley demonstrated that adding nickel to steel dramatically improved its toughness and hardness, sparking the industrial development of nickel-steel alloys that became crucial for armor plate and structural applications. The discovery of nickel's role as a cofactor in the enzyme urease by James Sumner, who received the 1946 Nobel Prize in Chemistry for crystallizing enzymes, was among the first demonstrations that nickel is biologically essential.

Nickel constitutes about 84 parts per million of Earth's crust by mass, making it the twenty-fourth most abundant crustal element, but its distribution in Earth as a whole is much greater because nickel is highly siderophilic — it tends to associate with iron — and a significant fraction of Earth's core is nickel, alloyed with iron at roughly 5 to 6 percent by mass. On the surface, nickel occurs primarily in two ore types: sulfide deposits, chiefly pentlandite ((Fe,Ni)9S8), found in ancient mafic and ultramafic igneous rocks in Canada's Sudbury Basin, Norilsk in Russia, and Western Australia; and laterite deposits, hydrated nickel silicates and oxides formed by prolonged tropical weathering of peridotite, which account for the majority of identified nickel resources. Deep-sea manganese nodules contain significant nickel concentrations. The largest producing nations are Indonesia, the Philippines, Russia, and Canada.

Nickel(II) sulfate (NiSO4) is the principal compound used in nickel electroplating baths and as a precursor in nickel catalyst preparation; its hexahydrate forms characteristic green crystals. Nickel(II) oxide (NiO) is used as a ceramic colorant producing gray and green hues, and as a precursor to other nickel compounds. Nickel tetracarbonyl (Ni(CO)4) is a toxic, volatile organometallic compound central to the Mond process: passing carbon monoxide over impure nickel at 50–60 degrees Celsius volatilizes it as Ni(CO)4, which decomposes back to pure nickel metal at 230 degrees Celsius, producing nickel of extraordinary purity. Nickel(II) chloride (NiCl2) serves as a catalyst in cross-coupling reactions and as a precursor in organometallic synthesis. Nickel hydroxide Ni(OH)2 is the positive electrode material in nickel-cadmium and nickel-metal hydride rechargeable batteries. In biochemistry, nickel appears in the active sites of several enzymes including urease, which cleaves urea to ammonia and carbon dioxide in bacteria, plants, and some invertebrates.

• The Mond process, patented in 1890, can purify nickel to 99.99 percent by converting it into the liquid compound nickel tetracarbonyl at low temperature and then decomposing it back to pure metal — a kind of chemical distillation of a solid metal.
• Earth's solid inner core is primarily iron with roughly 5 to 6 percent nickel, the same composition as iron-nickel meteorites called siderites, providing strong evidence that meteorites are fragments of differentiated planetary bodies similar to Earth.
• The US five-cent coin is called a 'nickel' but contains only 25 percent nickel alloyed with 75 percent copper; pure nickel coins were minted historically but the name stuck long after the composition changed.
• Nickel(II) ions in aqueous solution appear green because the octahedral complex [Ni(H2O)6]2+ absorbs red and yellow light, but add ammonia and the solution turns a striking violet-blue as water ligands are replaced by ammonia in [Ni(NH3)6]2+.
• Nickel is a known contact allergen — nickel dermatitis from belt buckles, jewelry, and watch clasps is one of the most common forms of allergic contact dermatitis in the world, affecting an estimated 10 to 20 percent of women and 1 to 2 percent of men.