Allotropy

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C
graphite
diamond
[60]fullerene
glassy carbon
N O
dioxygen
ozone
Si P
black phosphorus
white phosphorus
red phosphorus
S
α-sulfur
β-sulfur
γ-sulfur
ε-sulfur
Ge As
α-arsenic
yellow arsenic
ε-arsenic
amorphous arsenic
Se
α-selenium
β-selenium
γ-selenium
grey selenium
red selenium
black selenium
Sn
white tin
grey tin
Sb
α-antimony
antimony-I
antimony-II
Te
Pb Bi
α-bismuth
ζ-bismuth
Po
α-polonium
β-polonium
Some allotropes of the elements of groups 14, 15 and 16. The most stable allotrope at 25 °C and 1 bar is listed first; amorphous allotropes are listed in italics.

Allotropy is a phenomenon in chemistry whereby a given chemical element can exist in several different structural forms.[1] Each of these forms is called an allotrope. The best known example is that of diamond and graphite, which are both allotropes of carbon. Allotropy can be seen as a special case of polymorphism.

The term was first proposed by Jöns Jakob Berzelius in 1841:[2] it is derived from the Greek άλλοτροπἱα (allotropia; variation, changeableness), from ἄλλος (allos; other, different) and τρόπος (tropos; turn of mind, manner).[3]

At any given temperature and pressure, one allotrope will be more thermodynamically stable than the others. Nevertheless, other allotropes can exhibit substantial kinetic stability, e.g. diamond: such allotropes are said to be metastable.[4] Allotropy is most pronounced in groups 14, 15 and 16 of the periodic table. Boron also forms several allotropes: the different structural forms of the d- and f-block metals are usually referred to as phases rather than allotropes.

Nomenclature

Most allotropes have long-accepted semisystematic names (e.g., red phosphorus, white phosphorus) or trivial names (e.g., ozone, graphite). The different allotropes may also be labelled with Greek letters (e.g., α-sulfur, β-sulfur), where the α-allotrope is usually (but not always) the most stable under ambient conditions.

A systematic nomenclature is available, but is not widely used. Molecular allotropes are named using compositional nomenclature (e.g., tetraphosphorus for white phosphorus), while solid-state allotropes with infinite structures are labelled withe the Pearson symbol in parentheses after the element name (e.g., phosphorus(oS8) for black phosphorus).[5]

References

  1. allotropes, <http://goldbook.iupac.org/A00243.html> (accessed 5 April 2010), Compendium of Chemical Terminology Internet edition; International Union of Pure and Applied Chemistry (IUPAC).
  2. Jensen, W. B. The Origin of the Term Allotrope. J. Chem. Educ. 2006, 83 (6), 838–39. DOI: 10.1021/ed083p838.
  3. allotropy. In A New English Dictionary on Historical Principles; Oxford University Press, 1888; Vol. 1, p 238.
  4. metastability (of a phase), <http://goldbook.iupac.org/M03871.html> (accessed 5 April 2010), Compendium of Chemical Terminology Internet edition; International Union of Pure and Applied Chemistry (IUPAC).
  5. Nomenclature of Inorganic Chemistry; IUPAC Recommendations 2005; Royal Society of Chemistry: Cambridge, 2005; pp 49–51. ISBN 0-85404-438-8, <http://www.iupac.org/publications/books/rbook/Red_Book_2005.pdf>.

External links

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