'''Allotropy''' (Gr.
allos, other, and
tropos, manner), a name
applied by
Jöns Jakob Berzelius to the property possessed by certain
substances of existing in forms with different chemical structures; the various forms are known as
allotropes. Jöns Jakob Berzelius used the name in an entirely different sense (see Macmillan Encyclopedia of Chemistry, edited by J.J.Lagowski, 1997, Simon Schuster).
Some classic examples of elements that have allotropes are
phosphorus (in "red", "white", "purple" etc. forms),
oxygen (O
2 and O
3) and
carbon (in the form of
graphite,
diamond, fullerenes, and others - see
allotropes of carbon). The term allotropes may also be used to refer to the molecular forms of an element (such as a diatomic gas), even if there is only one such additional form.
Sulfur is an additional example of an element with several allotropic forms. Amorphous (plastic sulfur) is produced by quickly cooling the crystalline form, generating helical structure with eight atoms per spiral.
Allotropy specifically refers to the chemical bond structure between atoms of the same kind and should not be confused with the existence of multiple physical states, such as with
water, which can exist as a
gas (
steam), a
liquid (water), or a
solid (
ice). These phases of water are not allotropes, since they are caused by changes in the physical bonding between water molecules, rather than changes in the chemical bonding of the water molecules themselves. Allotropes of an element can be in any state, gaseous, liquid, or solid.
Allotropy usually refers to pure elemental solids, while
polymorphism may refer to elemental solids or more generally to any material having multiple crystal structures.
As can be seen with the example of carbon allotropes, certain physical properties can vary dramatically from allotrope to allotrope. In diamond, carbon
atoms are connected each to four other carbon atoms in a
tetrahedral lattice structure, whereas in graphite, each carbon atom is firmly bonded to just three other carbon atoms in
hexagonal sheets. These hexagonal sheets are then more loosely coupled to one another in stacks. The structure of fullerenes (a
carbon allotrope found in
soot) resembles that of graphite, except that instead of hexagons of carbon atoms, smaller regular
polygons are formed, such as a mix of hexagons and
pentagons, such that the sheet can fold back onto itself into closed
spheroids, as with the seams of a soccer ball.
Allotropes not only show dramatic differences in physical properties but also
show differences in chemical properties. Graphite can be oxidized by
nitric acid to give compounds related to
benzene whereas diamond does not give compounds related to benzene.
See also
Polymorph of crystal
Allotropes of carbon
Category:Inorganic chemistry
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