Allotropy

Allotropy (Gr. allos, other, and tropos, manner) is the name applied by Jöns Jakob Berzelius to the property possessed by specific pure elemental substances that can exist with different crystalline structures; the various forms are known as allotropes.

Definition

Allotropy is the ability of a chemical to exhibit a number of different and physically distinct forms in its pure elemental state. Carbon, for instance can exist as graphite, diamond and fullerene. 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. 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.

Allotropes should not be confused with changes of state/phase or isomers.

Differences in physical properties

On a nanoscopic level, the structure of allotropic forms can be drastically different. As such, their macroscopic properties may be very different too. To use the same example of carbon allotropes, diamond forms a tetrahedral lattice structure. As such, it is highly crystalline, has a high transmittance and is very hard on a macroscopic level. Graphite on the other hand forms broad flat sheets of hexagonal carbon rings with a conjugated electronic structure. These sheets are only weakly bonded together and are more or less free to slide past each other. As a result, graphite is semiconductive, has a negligible transmittance and is very soft. Graphite is often used in lubricants. Finally, fullerenes (and their derivatives, nanotubes) are a molecular allotrope and have chemical properties altogether different to any other form of carbon.

Allotropes tend to be affected by pressure and temperature, and many will only be stable given the correct conditions. For instance, iron only changes from ferrite to austenite above 723°C.

Examples

Some other good examples of allotropes include:

Phosphorus:

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

Oxygen:

dioxygen, O₂ - colourless gas
ozone, O₃ - pale blue gas/deep blue liquid
tetraoxygen, O₄ - red solid

Sulphur:

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

Carbon too has several additional lesser known allotropes - see allotropes of carbon.

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Definition

Differences in physical properties

Examples

See also