Properties

The electronic configuration of isolated nickel atom is counterintuitive: direct investigation^[3] finds that the predominant electron structure of nickel is [Ar] 4s¹ 3d⁹, which is the more stable form because of relativistic effects. Whereas Hund's rule, which works well for most other elements, predicts an electron shell structure of [Ar] 3d⁸ 4s² (the symbol [Ar] refers to the argon-like core structure). This latter configuration is found in many chemistry textbooks and is also written as [Ar] 4s² 3d⁸, to emphasize that the 3d shell is the electron shell being filled by the highest-energy electrons.

Physical

Nickel is a silvery-white metal with a slight golden tinge that takes a high polish. It is one of only four elements that are magnetic at or near room temperature. Its Curie temperature is 355 °C. That is, nickel is non-magnetic above this temperature.^[4] The unit cell of nickel is a face centered cube with the lattice parameter of 0.352 nm giving an atomic radius of 0.124 nm. Nickel belongs to the transition metals and is hard and ductile.

Isotopes

Main article: Isotopes of nickel

Naturally occurring nickel is composed of 5 stable isotopes; ⁵⁸Ni, ⁶⁰Ni, ⁶¹Ni, ⁶²Ni and ⁶⁴Ni with ⁵⁸Ni being the most abundant (68.077% natural abundance). ⁶²Ni is the most stable known nuclide of all the existing elements, even exceeding the stability of ⁵⁶Fe. 18 radioisotopes have been characterised with the most stable being ⁵⁹Ni with a half-life of 76,000 years, ⁶³Ni with a half-life of 100.1 years, and ⁵⁶Ni with a half-life of 6.077 days. All of the remaining radioactive isotopes have half-lives that are less than 60 hours and the majority of these have half-lives that are less than 30 seconds. This element also has 1 meta state.^[5]

Nickel-56 is produced by Silicon burning process and later set free in large quantities in type Ia supernovae and the shape of the light curve of these supernovae corresponds to the decay via beta radiation of nickel-56 to cobalt-56 and then to iron-56. Besides Nickel-56 also the stable Nickel isotopes.^[6] Nickel-59 is a long-lived cosmogenic radionuclide with a half-life of 76,000 years. ⁵⁹Ni has found many applications in isotope geology. ⁵⁹Ni has been used to date the terrestrial age of meteorites and to determine abundances of extraterrestrial dust in ice and sediment. Nickel-60 is the daughter product of the extinct radionuclide ⁶⁰Fe, which decays with a half-life of 2.6 million years. Because ⁶⁰Fe has such a long half-life, its persistence in materials in the solar system at high enough concentrations may have generated observable variations in the isotopic composition of ⁶⁰Ni. Therefore, the abundance of ⁶⁰Ni present in extraterrestrial material may provide insight into the origin of the solar system and its early history. Nickel-62 has the highest binding energy per nucleon of any isotope for any element (8.7946 Mev/nucleon).^[7] Isotopes heavier than ⁶²Ni cannot be formed by nuclear fusion without losing energy. Nickel-48, discovered in 1999, is the most proton-rich heavy element isotope known. With 28 protons and 20 neutrons ⁴⁸Ni is "double magic" (like ²⁰⁸Pb) and therefore unusually stable.^[8][5]

The isotopes of nickel range in atomic weight from 48 u (⁴⁸Ni) to 78 u (⁷⁸Ni). Nickel-78's half-life was recently measured to be 110 milliseconds and is believed to be an important isotope involved in supernova nucleosynthesis of elements heavier than iron.^[9]

Chemical

See also: Category:Nickel compounds

Nickel sulfate crystals

Tetracarbonyl nickel

The most common oxidation state of nickel is +2, but compounds of Ni⁰, Ni⁺, and Ni³⁺ are well known, and Ni⁴⁺ has been demonstrated.^[10]

Nickel(0)

Tetracarbonylnickel (Ni(CO)₄), discovered by Ludwig Mond,^[10] is a volatile liquid at room temperature. On heating, the complex decomposes back to nickel and carbon monoxide:

Ni(CO)₄ Ni + 4 CO

This behavior is exploited in the Mond process for purifying nickel, as described above.^[11] The related nickel(0) complex bis(cyclooctadiene)nickel(0) is a useful catalyst in organonickel chemistry due to the easily displaced cod ligands.

Nickel(II)

Nickel(II) compounds are known with all common anions, i.e. the sulfide, sulfate, carbonate, hydroxide, carboxylates, and halides. Nickel(II) sulfate is produced in large quantities by dissolving nickel metal or oxides in sulfuric acid. It exists as both a hexa- and heptahydrates.^[12] This compound is useful for electroplating nickel. The four halogens form nickel compounds, all of which adopt octahedral geometries. chloride is of particular significance, and its behavior is illustrative of the other halides. Nickel(II) chloride is produced by dissolving nickel residues in hydrochloric acid. The dichloride is usually encountered as the green hexahydrate, but it can be dehydrated to give the yellow anhydrous NiCl₂. Some tetracoordinate nickel(II) complexes form both tetrahedral and square planar geometries. The tetrahdral complexes are paramagnetic and the square planar complexes are diamagnetic. This equilibrium as well as the formation of octahedral complexes contrasts with the bahavior of the divalent complexes of the heavier group 10 metals, palladium(II) and platinum(II), which tend to adopt only square-planar complexes.^[10]

Nickel(III)

Nickel(III) oxide is used as the cathode in many rechargeable batteries, including nickel-cadmium, nickel-iron, nickel hydrogen, and nickel-metal hydride, and used by certain manufacturers in Li-ion batteries.