Allotropy

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Allotropy or allotropism (from the Greek ????? (allos) , meaning 'other', and ?????? (tropos) , meaning 'manner, form') is the nature of several chemical elements present in two or more different forms, in the same physical state, known as allotropes from this. element. Allotropes are different structural modifications of an element; the atomic elements are bonded together in different ways. For example, carbon allotropes include diamonds (carbon atoms bonded together in a tetrahedral lattice arrangement), graphite (carbon atoms bound together in hexagonal lattice sheets), graphene (single sheet graphite), and fullerenes (carbon atoms bound together in round, tubular, or ellipsoidal). The term allotropy is used for only elements, not for compounds. A more general term, used for any crystalline material, is polymorphism. Allotropy refers only to various forms of elements in the same phase (ie different forms of solid, liquid or gas); these different countries are not, are considered examples of allotropy.

For some elements, allotropes have different molecular formulas that can survive in different phases - for example, two allotropes of oxygen (oxygen, O ₂ , and ozone, O ₃ ), both can exist in dense, liquid and gas states. In contrast, some elements do not retain different allotropes in different phases - for example phosphorus has many solid allotropes, all of which return to the same P ₄ form when it melts into a liquid state.

Video Allotropy

Histori

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron JÃÆ'¶ns Jakob Berzelius (1779-1848). This term comes from the Greek word ?????????? (allotropia) , which means 'variability, change'. After the acceptance of Avogadro's hypothesis in 1860, it was understood that elements could exist as polyatomic molecules, and two allotropes of oxygen were recognized as O ₂ and O ₃ . At the beginning of the 20th century, it was recognized that other cases such as carbon were caused by differences in crystal structure.

In 1912, Ostwald noted that allotropy of elements is only a special case of polymorphism phenomena known for compounds, and proposes that the terms allotropes and allotropies are abandoned and replaced by polymorphs and polymorphisms. Although many other chemists have repeated this suggestion, IUPAC and most chemical texts still support the use of allotropes and allotropy only for elements.

Maps Allotropy

Differences of properties of allotropic elements

Allotropes are different structural shapes of the same element and can exhibit very different physical and chemical behavior. The change between the allotropic forms is triggered by the same forces that affect other structures, namely pressure, light, and temperature. Therefore, certain allotropic stability depends on certain conditions. For example, the change of iron from a body-centered (ferrite) cubic structure to an austenite cube structure above 906 ° C, and a tin undergoing a modification known as a lead pig from a metal form to a semiconductor form below 13.2 Ã, Â ° C (55,8Ã, Â ° F). For example allotropes have different chemical behavior, ozone (O ₃ ) is a much stronger oxidizer than dioxygen (O ₂ ).

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List of allotropes

Typically, elements capable of variable and/or oxidation coordination numbers tend to show larger numbers of allotropic shapes. Another contributing factor is the ability of elements to be attacked.

Examples of allotropes include:

Not metal

Metalloids

Metal

Among the elements of metal occurring in nature in significant amounts (56 to U, without Tc and Pm), nearly half (27) are allotropic at ambient pressure: Li, Be, Na, Ca, Ti, Mn, Fe, Co, Sr, Y, Zr, Sn, Gd, Tb, Dy, Yb, Hf, Tl, Th, Pa and U. Some transition phases between allotropic forms of relevant technology are metals Ti at 882 ° C, Fe at 912 ° C and 1394 ° C, Co at 422 ° C, Zr at 863 ° C, Sn at 13 ° C and U at 668 ° C and 776 ° C C.

Lanthanides and actinides

• Serium, samarium, dysprosium and ytterbium have allotropes.
• Praseodymium, neodymium, gadolinium and terbium have two allotropes.
• Plutonium has six different solid allotropes under "normal" pressure. Their density varies in a ratio of about 4: 3, which greatly complicates all types of work with metals (especially casting, machining, and storage). A seventh allotropes of plutonium exist at very high pressures. Transuranium metal Np, Am, and Cm are also allotropic.
• Promethium, americium, berkelium, and californium each have three allotropes.

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Nanoallotropes

In 2017, the concept of nanoallotropy was proposed by Prof Rafal Klajn of the Department of Organic Chemistry of the Weizmann Institute of Science. Nanoallotropes, or allotropes of nanomaterials, are nanoporous materials that have the same chemical composition (eg, Au), but differ in their architecture at the nanoscale (ie, on a scale of 10 to 100 times the dimensions of individual atoms). Such nanoallotrops can help create ultra-small electronic devices and find other industrial applications. Different nano architectures are translated into different properties, as shown for surface-enhanced Raman scattering performed on different gold nanoallotropes. A two-step method for producing nanoallotrop is also made.

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See also

• Isomer
• Polymorphism (material science)
• Allotropes of carbon superdense

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Note

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References

• Ã, Chisholm, Hugh, ed. (1911). "Allotropy". EncyclopÃÆ'Â|dia Britannica (issue 11). Cambridge University Press.

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External links

• "Untitled". web.archive.org. Archived from original on January 31, 2008 . Retrieved January 6, 2017 . CS1 maint: BOT: unknown original status-url (link)
• Allotropes - Chemical Encyclopedia

Source of the article : Wikipedia