Nickel
| Nickel | |
|---|---|
| |
| Atomic number | 28 |
| Symbol | Ni |
| Group | 10 |
| Electronegativity | 1.91 (Pauling) |
| Electron configuration | [Ar] 3d8 4s2 |
| Melting point | 1455 °C |
| Period | 4 |
| Phase STP | Solid |
| Block | d |
| Oxidation states | 0, +2, +3 |
| Wikidata | Q744 |
Nickel (symbol Ni, atomic number 28) is a hard, silvery-white metal in the transition-metal series. It belongs to group 10 of the periodic table (period 4, d‐block) and is solid at standard conditions. Nickel is famous for its toughness, high strength, and especially for its excellent corrosion resistance (it forms a thin protective oxide film). These properties make it valuable in coins, stainless steel alloys, protective platings, catalytic converters, and battery components. In chemistry, nickel most commonly appears in the +2 oxidation state (Ni²⁺), though metallic nickel itself is the uncharged (0) state and some compounds can reach +3.
Overview and Key Facts
Nickel is a transition metal, meaning its atoms have partially filled d-electron shells important for bonding chemistry. It has atomic weight about 58.69 u. In bulk form nickel is a silvery-white solid that is tougher and harder than iron, and it is ductile and malleable. At normal temperatures Nickel is ferromagnetic, meaning it can become a permanent magnet (its Curie temperature is about 627 K, above which it loses strong magnetism). Common oxidation states are +2 (most stable in compounds), followed by +3 or +1 in special cases. As an element, Ni at standard temperature and pressure is a solid metal.
Nickel’s combination of hardness, resistance to oxidation, and good conductivity of heat and electricity (it ranks in conductivity between base metals like iron and noble ones like copper) explains its many uses. For example, nickel plating of other metals protects them from corrosion, and nickel’s resistance to strong acids is why it is alloyed into stainless steel to help form a persistent chromium-nickel oxide layer on the surface.
Atomic Structure and Electron Configuration
A nickel atom has 28 protons and 28 electrons. The electron configuration can be written as [Ar] 3d⁸ 4s², meaning nickel has the noble-gas core of argon and ten electrons in its outer shells (two in the 4s orbital and eight in the 3d orbitals). These outer d and s electrons are the “valence” electrons that participate in chemical bonding. In its most stable Ni²⁺ state, nickel loses the two 4s electrons, leaving a 3d⁸ configuration.
In periodic trends, nickel is smaller in atomic radius than elements to its left (like iron) because of its higher nuclear charge pulling electrons in. The covalent radius of Ni is about 124 picometers (1.24 × 10⁻¹⁰ m). Its electronegativity on the Pauling scale is about 1.91, a moderate value that indicates nickel’s atoms attract bonding electrons fairly strongly for a metal (iron is about 1.83; copper about 1.90). The first ionization energy (energy to remove one electron) is around 737 kJ/mol.
These values mean nickel is less electropositive (more noble) than metals like zinc or iron, but less so than copper or silver. In practice, nickel will slowly dissolve in ordinary acids (releasing hydrogen gas), but much more slowly than zinc or iron, because its positive reduction potential means it holds electrons relatively tightly.
Isotopes and Nuclear Properties
Naturally occurring nickel is a mixture of five stable isotopes, with mass numbers 58, 60, 61, 62, and 64. Their natural abundances are roughly Ni-58 (68.1%), Ni-60 (26.2%), Ni-61 (1.1%), Ni-62 (3.6%), and Ni-64 (0.9%). (All stable nickel isotopes have even numbers of protons and neutrons except Ni-61, and most of them have nuclear spin zero, making them nonmagnetic at the nuclear level.) Notably, ^62Ni has the highest binding energy per nucleon of any nucleus, which is why very heavy elements beyond it cannot be formed by nuclear fusion without releasing energy.
Nickel has several radioisotopes made artificially or in cosmic processes. The longest-lived is Ni-59, which decays by beta emission with a half-life of about 76,000 years. Ni-59 is produced in supernovae and cosmic-ray spallation and can be used in isotope dating of meteorites or geological samples. Another useful radioisotope is Ni-63 (half-life ~100 years), produced by neutron irradiation of Ni, which emits low-energy beta particles; Ni-63 is used in specialized power sources (radioisotope batteries) and in some radiation detectors. A short-lived isotope Ni-56 (half-life 6 days) is created abundantly in supernovae; its decay (to cobalt-56 then iron-56) powers the visible light of stellar explosions. Altogether, about 19 nickel radioisotopes are known, but all beyond Ni-63 have half-lives less than a few years (most seconds or less).
Allotropes and Typical Compounds
Nickel has no distinct allotropes like carbon or phosphorus; it exists in essentially one solid form under ordinary conditions. That form is a face-centered cubic (FCC) metal crystal. No alternative crystal forms (allotropes) are stable at room temperature and pressure.
Nickel forms many compounds, most often in the +2 oxidation state. Common examples include:
- Oxides: NiO (black or green solid), Ni(OH)₂ (green), and mixed compounds like Ni₂O₃ or NiOOH in batteries. Nickel(II) oxide (NiO) is a green crystalline solid used in ceramics and some catalysts. A fully oxidized Ni(IV) state exists in materials like LiNiO₂ used in batteries, but pure NiO₂ (Ni in +4) is not stable alone.
- Hydroxides: Ni(OH)₂ is a pale green solid, and oxidized nickel oxyhydroxides (NiOOH) are dark green; these appear in alkaline batteries. In strong base, Ni(OH)₂ slowly dissolves by forming complex ions.
- Halides: Nickel forms NiF₂, NiCl₂, NiBr₂, and NiI₂. These are typically blue-green or yellow-green solids. For example, NiCl₂·6H₂O is a common green salt (used in electroplating). In solution, Ni(II) salts often appear green or blue.
- Sulfides and Sulphates: NiS (a black crystal), Ni₃S₂ (millerite mineral) and other sulfides occur in nature. Nickel sulfate (NiSO₄) is a blue-green solid widely used in electroplating baths.
- Carbonyl: A striking compound is nickel carbonyl Ni(CO)₄, a colorless volatile liquid formed by reacting nickel with carbon monoxide under pressure (the Mond process). Ni(CO)₄ is extremely toxic but historically important for purifying nickel metal.
- Organometallics: Nickel forms a variety of coordination complexes, such as [Ni(CN)₄]²⁻ (square-planar, actually blue) or [Ni(NH₃)₆]²⁺ (octahedral, blue). In these, nickel retains a +2 oxidation state. Nickel(I) or (III) complexes are rare but do exist under some conditions.
- Hydrides: Nickel does not form stable simple hydrides at ambient conditions. However, nickel-metal alloys can absorb hydrogen (as in Ni-MH batteries) by forming metal hydrides under pressure. Also, very high-pressure nickel hydrides (like NiH) have been observed in specialized research.
Typical bonding patterns: In solids, nickel atoms bond by metallic bonding, sharing electrons in a lattice. In compounds, Ni(II) commonly forms octahedral complexes (coordination number 6) because 3d⁸ favors that geometry (for example [Ni(H₂O)₆]²⁺ is green). Some nickel compounds adopt square-planar geometry (especially with cyanide or other strong-field ligands). Nickel also forms covalent bonds in organometallics (as in Ni(CO)₄ or Ni(PPh₃)₂(C₂H₄) complexes).
Physical Properties
Nickel is a heavy metal with density about 8.9 g/cm³ at room temperature (roughly 8900 kg/m³). It is a hard, tough metal (Mohs hardness ≈4) that is more durable than many common metals. The melting point is high — about 1455 °C — and boiling point around 2730 °C, reflecting the strong metallic bonds in its crystal. At standard conditions Ni is a silvery lustrous solid that tarnishes slowly in air (forming a dull oxide layer).
Structurally, nickel crystallizes in a face-centered cubic (FCC) lattice. In this arrangement each atom has 12 nearest neighbors. Pure nickel remains FCC up to its melting point. (Under very high pressures, some experiments suggest possible transformations to other structures, but these are only at extreme laboratory conditions.)
Thermally and electrically, nickel is a good conductor: its thermal conductivity is about 90 W/(m·K) (lower than copper’s ~400, but still high for a metal), and its electrical resistivity at 20°C is on the order of 7 × 10⁻⁸ Ω·m (hence good electrical conductor). Nickel’s magnetic behavior is notable: it is ferromagnetic at room temperature, meaning domains of aligned electron spins give it a permanent magnetic field. The Curie temperature (above which it becomes merely paramagnetic) is about 627 K (354 °C). In paramagnetic form (above that point), nickel still has a high relative magnetic permeability compared to non-magnetic metals.
Spectroscopically, nickel atoms have many lines in the ultraviolet and visible region, but there’s no single “flame test” color (nickel compounds often burn a green flame). One could note that nickel’s electronic transitions produce greenish and blue colors in its complexes (for example nickel chloride or sulfate solutions are often green/blue). In astronomy, lines of neutral Ni or ions are sometimes used in stellar spectroscopy, but nickel has no brief mnemonic identifier like sodium’s D-line.
Chemical Reactivity and Trends
Nickel metal is moderately reactive. It does not react with water or air at ambient conditions (owing to its protective oxide surface), but it will slowly oxidize at higher temperatures or in the presence of strong oxidizers. In the activity series, nickel lies below iron but above copper. That means nickel is less chemically reactive than iron (it doesn’t rust easily) but more reactive than copper (it will oxidize more readily). For example, nickel will dissolve slowly in hydrochloric or sulfuric acid, liberating hydrogen gas (H₂), whereas copper won’t react with these acids without an oxidizer. It does not react with water or oxygen at room temperature. Hot concentrated nitric acid or aqua regia will attack nickel metal because these oxidizing acids can convert Ni to Ni²⁺.
As a metal, nickel tends to passivate: it forms a thin oxide or hydroxide layer on its surface that shields the underlying metal from further corrosion. This is why nickel and nickel-containing stainless steels are so corrosion-resistant. Nickel in alloys helps form protective chromium-oxide or nickel-oxide layers. Nickel also remains relatively stable in alkali; it resists alkali attack unless strong complexing agents are present.
In acids and bases, nickel chemistry is characterized by the Ni²⁺ ion. Most nickel salts are blue-green in water. For example, nickel(II) chloride or nitrate solutions are greenish. The Ni²⁺ ion easily forms octahedral aqua complexes [Ni(H₂O)₆]²⁺. Adding an alkali (like NaOH) precipitates nickel(II) hydroxide Ni(OH)₂, a green solid. Some nickel salts are slightly soluble in ammonia or complexing agents: for instance, ammonia converts the green Ni²⁺ solution to [Ni(NH₃)₆]²⁺, which is deep blue. Nickel coordination complexes are moderately stable, stronger than many alkaline-earth or post-transition metals but weaker than those of first-row metals like Fe or Cu in similar oxidation states.
Nickel shows some higher oxidation states in oxides or complexes. For instance, in strongly oxidizing conditions, Ni²⁺ can be oxidized to Ni³⁺ (as in nickel oxyhydroxide NiOOH, found in NiCd battery electrodes). A +4 state can appear in solid oxides (as in layered LiNiO₂ battery cathodes, read as Ni~IV partially). However, unlike iron or copper, nickel rarely exhibits higher states beyond +2 in simple compounds; +1 is also uncommon and confined to special cluster compounds or organometallics.
Nickel metal also acts as a catalyst in redox reactions. On the periodic table, nickel’s position (group 10) places it below palladium and platinum, which are famous catalysts, and sure enough nickel serves as a cheaper hydrogenation catalyst. In catalytic hydrogenation, nickel surfaces adsorb H₂ and organic molecules, facilitating addition of hydrogen to double bonds (e.g., converting vegetable oils to margarine). “Raney nickel” is a porous finely divided form of Ni (made by treating some Ni alloy with sodium hydroxide) that provides a very large surface area catalyst. Nickel oxide and nickel sulfide also catalyze certain oxidation or reforming reactions in petroleum refining. In acid-base terms, nickel compounds are mildly amphoteric (Ni(OH)₂ will dissolve in excess strong alkali, forming [Ni(OH)₄]²⁻ complexes, though this is not highly pronounced).
In terms of electron-transfer (redox) behavior, the standard reduction potential for Ni²⁺/Ni is about –0.26 V (versus the hydrogen electrode). This negative value indicates that Ni has a tendency to stay metallic until quite strong acids are present. By comparison, copper’s potential is +0.34 V (meaning Cu²⁺ is less easily reduced to metal than Ni²⁺, so copper metal is more inert). Thus, nickel metal can oxidize to Ni²⁺ where hydrogen gas is evolved (e.g., in HCl), whereas copper will not.
Occurrence and Production
Nickel is fairly abundant in the solar system and in the Earth. In cosmic abundance, it is one of the heavier of the common elements produced in supernovae. On Earth’s crust it is about 0.008% by weight (similar to zinc). Much of Earth’s nickel resides in its core (as nickel-iron alloy), and in outer layers nickel is scattered in minerals. The most famous natural nickel occurrences are iron meteorites, which typically contain 5–20% nickel alloyed with iron; the presence of nickel is one clue to the meteoritic origin of such iron. Terrestrially, nickel is found in two main types of ore deposits:
- Sulfide ores: These include pentlandite ((Ni,Fe)₉S₈), millerite (NiS), and vaesite (NiS₂) often mixed with copper and iron sulfides. Such deposits occur in places like Sudbury (Canada), Norilsk (Russia), and the Great Lakes region of North America.
- Laterite (oxide) ores: Formed by weathering of nickel-bearing rocks in tropical climates, yielding mixtures like garnierite (nickel magnesium silicate) which contains nickel hydroxide and serpentinite. Major laterite deposits are in New Caledonia, Indonesia, Philippines, and parts of Australia.
World nickel production today comes largely from Indonesia and the Philippines (from tropical laterites), Russia, Canada, and Australia. After mining, nickel ore is processed differently depending on type. Sulfide ores are usually concentrated by flotation, then roasted or smelted to yield a nickel-iron alloy (“matte”) from which nickel is separated (often by converting to nickel oxide or sulfide and reducing it to metal). Laterites are processed either by high-pressure acid leaching (in which ore is treated with sulfuric acid under pressure, then precipitated) or by smelting to produce ferronickel.
A key refining step for high-purity nickel metal is the Mond (carbonyl) process: impure nickel is reacted with carbon monoxide at about 50 °C to form the volatile Ni(CO)₄, which is distilled away from contaminants and then decomposed at ~200 °C to yield pure nickel metal. Electrolytic refining is also used: nickel sulfate solutions are electrolyzed to deposit pure nickel onto cathodes.
Overall, global production of refined nickel metal is on the order of 2 million metric tons per year. A significant fraction of this comes from smelters in Canada (formerly Inco, now Vale), Norilsk (Russia), and from Indonesian/Philippines laterite operations. Nickel is also extensively recycled from stainless steel scrap and old batteries.
Applications and Technology
Nickel’s combination of properties has led to a wide realm of applications, especially where strength, high temperature performance, or corrosion resistance are needed. Key uses include:
- Stainless Steel and Alloys: Perhaps the largest single use of nickel is in stainless steels, where 8–25% Ni is added along with chromium to steel to form the characteristic corrosion-resistant alloy. Nickel improves ductility, toughness, and oxidation resistance of stainless steel at high temperatures. More exotic nickel-based superalloys (like Inconel and Hastelloy) with high nickel and chromium (plus other metals) are critical in jet engines, gas turbines, and chemical plant equipment for their heat resistance. Other important alloys: Monel (≈67% Ni–Cu) for marine and chemical duties; Invar (Fe–36% Ni) with nearly zero thermal expansion used in precision instruments; Permalloy (≈80% Ni–20% Fe) used for magnetic cores and recording heads; nickel–titanium (Nitinol) shape-memory alloy used in medical devices; and nickel–chromium alloys (nichrome) used in electrical resistors and heating elements.
- Electroplating and Surface Finishing: Nickel plating (electroplated Ni) is widely used to coat other metals or even plastics with a decorative, lubricious, and corrosion-resistant surface. Nickel-plated parts are found in automotive parts, kitchenware, plumbing fixtures, and electronics contacts. The plated layer also provides a good base for a final chrome or silver finish. Because nickel is hard and wear-resistant, nickel plating also hardens surfaces. Even without plating, bulk nickel parts can be stamped or formed into coins (nickel alloys are used in many currencies). The name “nickel” even comes up in coinage: e.g., the U.S. five-cent coin is often called a “nickel”, and many other modern coins use nickel-copper alloys.
- Catalysts: Finely divided nickel and nickel compounds are important hydrogenation catalysts. In the food industry, nickel catalysts (like Raney Ni) hydrogenate vegetable oils to make margarine. In petrochemistry, nickel catalysts are used for hydrogenating unsaturated hydrocarbons and for hydrodesulfurization (removing sulfur from fuels). Nickel is also used in some batteries as an electrode (for example, in Ni–Cd and Ni–MH batteries) both because of its reducibility and its ability to form oxide/hydroxide electrodes that store charge.
- Batteries and Energy Storage: Nickel plays a central role in several battery technologies. Older rechargeable nickel–cadmium (NiCd) batteries use a nickel oxide hydroxide cathode and a cadmium anode in an alkaline electrolyte. More common now are nickel–metal-hydride (NiMH) batteries, where the positive electrode is Ni(OH)₂ and the negative electrode is a metal-hydride alloy (often a lanthanum–nickel alloy). NiMH cells are widely used in portable electronics and hybrid vehicles (e.g. Toyota Prius). Nickel also appears in lithium-ion battery cathodes: modern high-performance cathodes often contain nickel (e.g. LiNiMnCoO₂ or LiNiCoAlO₂) because nickel enables a higher capacity. Nickel is also being explored in metal–air and fuel cells as an electrode catalyst (e.g. nickel foam electrodes for alkaline fuel cells).
- Electrical and Electronic Uses: Nickel’s high conductivity and resistance to corrosion make it useful in electronics. Nickel plating is used on connectors, circuit boards, and shielding. Nickel is also used as electrodes for quartz crystal oscillators (as a plating for the electrodes). In sensors, nickel forms the positive leg of the common NiCr–Ni (chromel–alumel) thermocouple (Type K). Nickel has some semiconductor applications (nickel silicide is used in CMOS processes).
- Chemical and Other Uses: Nickel compounds are used as pigments (nickel oxide green in ceramics and paints), in catalysts for hydrogen generation, and in various chemical reagent roles. Nickel also has niche uses, for example in magnetic recording media, as a catalyst support, in electrodes for electrolysis, and in supercapacitor electrodes.
Biology, Environment, and Safety
Biology: Nickel is an essential trace element for some microorganisms and plants. It is a crucial component of certain enzymes (for example, urease in plants, which breaks down urea, and NiFe-hydrogenase in bacteria). In animals and humans, no proven essential role is known, but small amounts (micrograms per day) are ingested normally in the diet.
Toxicity and Health: In larger doses, nickel can be harmful. Nickel allergy is very common: when nickel metal or soluble nickel salts contact the skin, many people develop contact dermatitis (itchy skin rash). This is why nickel is often plated over with a less allergenic metal or nickel-free alloys for jewelry. Inhalation of certain nickel dusts or fumes (such as nickel subsulfide or nickel carbonyl) is carcinogenic; chronic exposure can lead to lung and nasal cancers. Soluble nickel salts (e.g. NiCl₂, NiSO₄) are toxic if ingested in high enough amounts, though elemental nickel metal is much less readily absorbed. The lethal dose (oral) for nickel salts is on the order of hundreds of milligrams per kilogram of body weight (so very high doses are required), but occupational exposure limits (for inhalation) are on the order of 0.1–1 mg/m³. Nickel carbonyl (Ni(CO)₄) is extraordinarily toxic even at low concentrations and must be handled with extreme care; it was once used in industry but is now largely avoided due to safety.
Environmental: Nickel enters the environment from natural sources (erosion of rocks, volcanoes) and anthropogenic sources (burning fossil fuels, industrial emissions, nickel mining and smelting). In soil and water, nickel is moderately soluble especially in acidic conditions. It can accumulate in plants and microorganisms. Aquatic life is sensitive to nickel at levels above about 0.05–0.1 mg/L; for this reason many water agencies set guidelines of order 0.01–0.02 mg/L as a safe concentration for drinking water. Human drinking-water guidelines often recommend no more than ~0.02 mg/L of nickel. Occupational exposure is managed by ventilation and respiratory protection to avoid inhaling nickel dust or fumes. Care is taken in waste disposal of nickel-containing materials (like plating baths) to prevent environmental contamination.
Handling: Nickel metal itself is relatively safe to handle with minimal hazards (though dust and filings should be avoided). However, key safety notes include avoiding nickel carbonyl (too dangerous) and controlling exposure to nickel salts (gloves, masks). Any acid or caustic used to etch or clean nickel-containing alloys should be handled with standard precautions (gloves, goggles) because toxic nickel ions can dissolve into the solution.
History and Etymology
Nickel has a colorful history. Its ore and name came from mining lore. The German miners called the copper-colored ore Kupfernickel, meaning “copper demon” or “false copper,” because it looked like copper ore (Kupfer) but gave no copper when smelted; they blamed a mischievous goblin nickeln. In 1751, Swedish chemist Axel Fredrik Cronstedt studied this ore (later identified as niccolite, NiAs) and extracted a previously unknown white metal from it. Recognizing it was a new element, he named it nickel after the ore’s nickname.
Pure nickel metal was isolated over subsequent decades. In 1804 Hans Esmark noted nickel in iron meteorites. In 1828 William T. Brande and John F. Daniell (London) and independently Jöns Jacob Berzelius (Stockholm) identified nickel as a distinct element with a symbol (Ni). The industrial era of nickel began when Ludwig Mond invented the carbonyl purification method in 1890, allowing large-scale, high-purity nickel production.
Historical milestones include the introduction of nickel coinage (for example, the U.S. nickel coin in 1866 and British coinage containing nickel in 1860), and the development of nickel-based alloys. A famous patented nickel-iron alloy was Monel (1895), and nickel-chromium alloys (nichrome) powered early electrical resistors. Nickel–cadmium batteries were invented in the late 19th century but became widespread only in the 20th century. Nickel plating technologies emerged in the 19th century for coating wood and metal. In the mid-20th century, nickel’s role in stainless steel (discovered 1913) and aerospace superalloys (1940s onward) became crucial for modern technology.
The name "nickel" entered many languages, derived from the German Nickel. It was sometimes thought of as associated with St. Nicholas ("Nick’s copper") as well. But most importantly, nickel’s discovery helped shape modern chemistry by expanding knowledge of transition metals and by contributing to metallurgy. Its story from ore dubbed “devil’s copper” to an essential industrial metal is a classic tale of science taming nature’s seemingly mysterious substances.
Data Table
| Property | Value |
|---|---|
| Symbol | Ni |
| Atomic number (Z) | 28 |
| Standard atomic weight | 58.6934 (units atomic mass u) |
| Element category | Transition metal (d-block, Group 10, Period 4) |
| Electron configuration | [Ar] 3d⁸ 4s² |
| Common oxidation states | 0 (metal), +2 (most stable), +3 (less common) |
| Valence electrons | 10 total (3d⁸ 4s², often treated as 8 in d-shell) |
| Magnetic ordering | Ferromagnetic at room temperature (Curie 627 K) |
| Crystal structure (solid) | Face-centered cubic (FCC) |
| Atomic/covalent radius | ~124 pm (covalent); atomic radius ~135 pm |
| Electronegativity (Pauling) | 1.91 |
| First ionization energy | ~737 kJ/mol (7.64 eV) |
| Density (20 °C) | 8.90 g/cm³ (8900 kg/m³) |
| Melting point | 1455 °C |
| Boiling point | 2730 °C |
| Phase at STP | Solid metal |
| Stable isotopes | ^58Ni, ^60Ni, ^61Ni, ^62Ni, ^64Ni |
| Natural abundance (most) | ^58Ni ~ 68.1% |
| Discovery | 1751 by A. F. Cronstedt (Sweden) |
| Name origin | From German Kupfernickel (“devil’s copper”) |