Allotropy

Allotropy (Gr. allos, other, and tropos, manner), a name applied by Jons
Jacob Berzelius to the property possessed by certain substances of existing
in different modifications; the various forms are known as allotropes. Jones
Jacob 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" and "white" and "purple" etc. forms) and carbon (in the form of
graphite, diamond, or fullerenes). 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.

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 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.

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