Aluminium

Aluminium or aluminum is a chemical element; it has symbol Al and atomic number 13. It has a density lower than other common metals, about one-third that of steel. Aluminium has a great affinity toward oxygen, forming a protective layer of oxide on the surface when exposed to air. It visually resembles silver, both in its color and in its great ability to reflect light. It is soft, nonmagnetic, and ductile. It has one stable isotope, ²⁷Al, which is highly abundant, making aluminium the 12th-most abundant element in the universe. The radioactivity of ²⁶Al leads to it being used in radiometric dating.
Chemically, aluminium is a post-transition metal in the boron group; as is common for the group, aluminium forms compounds primarily in the +3 oxidation state. The aluminium cation Al³⁺ is small and highly charged; as such, it has more polarizing power, and bonds formed by aluminium have a more covalent character. The strong affinity of aluminium for oxygen leads to the common occurrence of its oxides in nature. Aluminium is found on Earth primarily in rocks in the crust, where it is the third-most abundant element after oxygen and silicon, rather than in the mantle, and virtually never as the free metal. It is obtained industrially by mining bauxite, a sedimentary rock rich in aluminium minerals.
The discovery of aluminium was announced in 1825 by Danish physicist Hans Christian Ørsted. The first industrial production of aluminium was initiated by French chemist Henri Étienne Sainte-Claire Deville in 1856. Aluminium became much more available to the public with the Hall–Héroult process developed independently by French engineer Paul Héroult and American engineer Charles Martin Hall in 1886, and the mass production of aluminium led to its extensive use in industry and everyday life. In 1954, aluminium became the most produced non-ferrous metal, surpassing copper. In the 21st century, most aluminium was consumed in transportation, engineering, construction, and packaging in the United States, Western Europe, and Japan. The standard atomic weight of aluminium is low in comparison with many other metals, giving it the low density responsible for many of its uses.
Despite its prevalence in the environment, no living thing is known to metabolize aluminium salts, but aluminium is well tolerated by plants and animals. Because of the abundance of these salts, the potential for a biological role for them is of interest, and studies are ongoing.

Physical characteristics

Isotopes

Aluminium has one stable isotope, ²⁷Al, which comprises virtually all of the naturally-occurring element. This is common for elements with an odd atomic number. It is therefore a mononuclidic element for standard atomic weight, which is determined completely by that isotope. Aluminium is useful in nuclear magnetic resonance, as its single stable isotope has a high NMR sensitivity.
All other isotopes of aluminium are radioactive. The most stable of these is ²⁶Al, with a half-life of 717,000 years. While it was present along with stable ²⁷Al in the interstellar medium from which the Solar System formed, no detectable amount could have survived the time since the formation of the planet. However, minute traces of ²⁶Al are still produced from decay of argon in the atmosphere induced by ionizing radiation of cosmic rays. The ratio of ²⁶Al to ¹⁰Be has been used for the radiodating of geological processes over 10⁵ to 10⁶ year time scales, in particular transport, deposition, sediment storage, burial times, and erosion. Most meteorite scientists believe that the energy released by the decay of ²⁶Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.
The other known isotopes of aluminium, with mass numbers ranging from 20 to 43, all have half-lives less than 7 minutes, as do the four detected metastable states.

Electron shell

An aluminium atom has 13 electrons with an electron configuration of, with three electrons beyond a stable noble gas configuration. Accordingly, the combined first three ionization energies of aluminium are far lower than the fourth ionization energy alone. Such an electron configuration is shared with the other well-characterized members of its group, boron, gallium, indium, and thallium; it is also expected for nihonium. Aluminium can surrender its three outermost electrons in many chemical reactions. The electronegativity of aluminium is 1.61 on the Pauling scale.
File:Aluminium_Atomic_lattice.png|alt=M. Tunes & S. Pogatscher, Montanuniversität Leoben 2019 No copyrights =)|left|thumb|High-resolution STEM-HAADF micrograph of Al atoms viewed along the zone axis.
A free aluminium atom has an atomic radius of 143 pm. With the three outermost electrons removed, the radius shrinks to 39 pm for a 4-coordinated atom or 53.5 pm for a 6-coordinated atom. At standard temperature and pressure, aluminium atoms form a face-centered cubic crystal system bound by metallic bonding provided by atoms' outermost electrons; hence, aluminium is a metal. This crystal system is shared by many other metals, such as lead and copper; the size of a unit cell of aluminium is comparable to that of those other metals. This system, however, is not shared by the other members of its group: boron has ionization energies too high to allow metallization, thallium has a hexagonal close-packed structure, and gallium and indium have unusual structures that are not close-packed like those of aluminium and thallium. The few electrons that are available for metallic bonding in aluminium are a probable cause for it being soft with a low melting point and low electrical resistivity.

Bulk

Aluminium metal has an appearance ranging from silvery white to dull gray depending on its surface roughness. Aluminium mirrors provides high reflectivity for light in the ultraviolet, visible, and far infrared regions. Aluminium is also good at reflecting solar radiation, although prolonged exposure to sunlight in air can deteriorate the reflectivity of the metal; this may be prevented if aluminium is anodized, which adds a protective layer of oxide on the surface.
The density of aluminium is 2.70 g/cm³, about one-third that of steel, much lower than other commonly encountered metals, making aluminium parts easily identifiable through their lightness. Aluminium's low density compared to most other metals arises from the fact that its unit cell size is relatively large in proportion to the number of nucleons. The only lighter metals are the metals of groups 1 and 2, which, apart from beryllium and magnesium, are too reactive for structural use. Aluminium is not as strong or stiff as steel, but the low density makes up for this in the aerospace industry and for many other applications where light weight and relatively high strength are crucial.
Pure aluminium is quite soft and lacking in strength. In most applications, various aluminium alloys are used instead because of their higher strength and hardness. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium is ductile, with a percent elongation of 50–70%, and malleable allowing it to be easily drawn and extruded; it is also easily machined and cast.
Aluminium is an excellent thermal and electrical conductor, and the amount of aluminium required to match the same amperage in copper weighs only half as much. Aluminium is capable of superconductivity, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss. It is paramagnetic and thus essentially unaffected by static magnetic fields. However, the high electrical conductivity means that it is strongly affected by alternating magnetic fields through the induction of eddy currents.

Chemistry

Aluminium combines characteristics of pre- and post-transition metals. Since it has few available electrons for metallic bonding, like the heavier group 13 elements, it has the characteristic physical properties of a post-transition metal, with longer-than-expected interatomic distances. Furthermore, as Al³⁺ is a small and highly charged cation, it is strongly polarizing, and bonding in aluminium compounds tends towards covalency; this behavior is similar to that of beryllium, displaying an example of a diagonal relationship.
The underlying core of electrons under aluminium's valence shell is that of the preceding noble gas, whereas those of the heavier group 13 elements gallium, indium, thallium, and nihonium also include a filled d-subshell and in some cases a filled f-subshell. Hence, the inner electrons of aluminium shield the valence electrons almost completely, unlike those of the heavier group 13 elements. As such, aluminium is the most electropositive metal in its group, and its hydroxide is in fact more basic than that of gallium. Aluminium also bears minor similarities to boron, which is in the same group: AlX₃ compounds are valence isoelectronic to BX₃ compounds, and both behave as Lewis acids and readily form adducts. Additionally, one of the main motifs of boron chemistry is regular icosahedral structures, and aluminium forms an important part of many icosahedral quasicrystal alloys, including the Al–Zn–Mg class.
Aluminium has a high chemical affinity to oxygen, which renders it suitable for use as a reducing agent in the thermite reaction. A fine powder of aluminium reacts explosively on contact with liquid oxygen; under normal conditions, however, aluminium forms a thin oxide layer that protects the metal from further corrosion by oxygen, water, or dilute acid, a process termed passivation. Aluminium is not attacked by oxidizing acids because of its passivation. This allows aluminium to be used to store reagents such as nitric acid, concentrated sulfuric acid, and some organic acids.
In hot, concentrated hydrochloric acid, aluminium reacts with water through evolution of hydrogen, and it reacts in aqueous sodium or potassium hydroxide at room temperature to form aluminates; protective passivation under these conditions is negligible. Aqua regia also dissolves aluminium. Aluminium is also corroded by dissolved chlorides, such as common sodium chloride. The oxide layer on aluminium is also destroyed by contact with mercury due to amalgamation or by contact with salts of some electropositive metals. As such, the strongest aluminium alloys are less corrosion-resistant due to galvanic reactions with alloyed copper, and aluminium's corrosion resistance is greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.
Aluminium reacts with most nonmetals upon heating, forming compounds such as aluminium nitride, aluminium sulfide, and the aluminium halides. It also forms a wide range of intermetallic compounds involving metals from every group on the periodic table.