Allotropy

Allotropy (Gr. allos, other, and tropos, manner) is a behaviour exhibited by certain chemical elements: these elements can exist in two or more different forms, known as allotropes of that element. In each different allotrope, the element's atoms are bonded together in a different manner.

For example, the element carbon has two common allotropes: diamond, where the carbon atoms are bonded together in a tetrahedral lattice arrangement, and graphite, where the carbon atoms are bonded together in sheets of a hexagonal lattice.

Note that allotropy refers only to different forms of an element within the same phase or state of matter (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 can persist in different phases - for example, the two allotropes of oxygen (dioxygen and ozone), 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 P₄ form when melted to the liquid state.

The term "allotrope" was coined by the famous chemist Jöns Jakob Berzelius.

Differences in properties of an element's allotropes

Allotropes of the same element can typically exhibit quite different physical properties and chemical behaviour, even though they contain nothing else but atoms of that element. They may have different colours, odour, hardnesses, electrical and thermal conductivies, etc.

The change between different allotropic forms of an element is often triggered by pressure and temperature, and many allotropes are only stable in the correct conditions. For instance, iron only changes from ferrite to austenite above 723°C, and tin undergoes a process known as tin pest at 13.2°C and below.

Examples of allotropes

Typically, elements capable of variable coordination numbers 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 and metalloids.

Examples of allotropes include:

Carbon:

diamond - an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor conductor.
graphite - a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices, which are then layered in sheets.
fullerene - (including the "buckyball", C₆₀)

Phosphorus:

Red Phosphorus - polymeric solid
White Phosphorus - crystalline solid
Black Phosphorus - semiconductor, analogous to graphite

Oxygen:

dioxygen, O₂ - colourless
ozone, O₃ - blue
tetraoxygen, O₄ - red

Sulfur:

Plastic (amorphous) sulfur - polymeric solid
Rhombic sulfur - large crystals composed of S₈ molecules
Monoclinic sulfur - fine needle-like crystals
Molecular sulfur - sulphur tends to form ring molecules such as S₇ and S₁₂