311Allotropes Section
Allotropy or allotropism is the property of some chemical elements to exist in two or more
different forms, known as allotropes of these elements. Allotropes are different structural
modifications of an element; the atoms of the element are bonded together in a different
manner.
For example, the allotropes of carbon include diamond (where the carbon atoms are
bonded together in a tetrahedral lattice arrangement), graphite (where the carbon atoms
are bonded together in sheets of a hexagonal lattice), graphene (single sheets of
graphite), and fullerenes (where the carbon atoms are bonded together in spherical,
tubular, or ellipsoidal formations).
The term allotropy is used for elements only, not for compounds. The more general term,
used for any crystalline material, is polymorphism. Allotropy refers only to different forms
of an element within the same phase (i.e. different solid, liquid or gas forms); the changes
of state between solid, liquid and gas in themselves are not considered allotropy.
For some elements, allotropes have different molecular formulae which can persist in
different phases – for example, two allotropes of oxygen (dioxygen, O 2 and ozone, O 3 ),
can both exist in the solid, liquid and gaseous states.
Conversely, some elements do not maintain distinct allotropes in different phases – for
example phosphorus has numerous solid allotropes, which all revert to the same P 4 form
when melted to the liquid state.
The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron
Jöns Jakob Berzelius (1779–1848). The term is derived from the Greek άλλοτροπἱα
(allotropia; variability, changeableness).
After the acceptance of Avogadro's hypothesis in 1860 it was understood that elements
could exist as polyatomic molecules, and the two allotropes of oxygen were recognized
as O 2 and O 3. In the early 20th century it was recognized that other cases such as carbon
were due to differences in crystal structure.
By 1912, Ostwald noted that the allotropy of elements is just a special case of the
phenomenon of polymorphism known for compounds, and proposed that the terms
allotrope and allotropy be abandoned and replaced by polymorph and polymorphism.
Although many other chemists have repeated this advice, IUPAC and most chemistry texts
still favor the usage of allotrope and allotropy for elements only.
List of Allotropes
Typically, elements capable of variable coordination number and/or oxidation states tend
to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of
an element to catenate. Allotropes are typically more noticeable in non-metals (excluding
the halogens and the noble gases) and metalloids. Nevertheless, metals tend to have
many allotropes.