What are allotropes? Simply put, an allotrope is one of a variety of forms in which an element can exist. This does not refer merely to the state of an element, whether solid, liquid, gas, or plasma; it refers to the tendency of an element to engage in “other behavior,” that is to exist in a variety of manifestations.
Thus carbon can be found in a variety of forms¹ of graphite, charcoal, diamond, and fullerenes.
Sulfur allotropes exist as an assortment of structural forms, including rings of atoms ranging from six to twenty atoms. Especially well known are the monoclinic and rhombic crystalline forms.
Phosphorus allotropes exist in a variety of colors. Ordinary phosphorous is called white phosphorous (P₄); it is unstable under ordinary conditions, and dangerous to come into contact with. There are red, violet, and black forms of phosphorous, as well. These forms possess increased stability.
Breaking one or more bonds and modifying the connections between them leads to such other forms of phosphorous. Curiously, the black form consists of interconnected hexagonal rings, similar in structure to the graphite form of carbon.
Even oxygen, the most important element in air for the maintaining of life, exists in a variety of allotropic forms. Ordinary oxygen exists as diatomic molecules (O₂). The same element, existing as groups of three atoms, makes up ozone (O₃). Tetraoxygen (the unstable O₄) and Octaoxygen (O₈) are not unknown.
Selenium & the Metalloids
There are red, gray, and black forms of selenium. Metalloids are known to exist in a variety of allotropes. Among the elements exhibiting allotropy are boron, silicon, arsenic, germanium, and antimony.
Compounds display properties somewhat similar to allotropy, but these are said to exhibit polymorphism. Unlike allotropy, there are a host of forms of polymorphism—too many to discuss here—all of them of great interest to the materials scientist.
¹ Some do not like to use the word form to describe allotropic behavior. It is a convenient word, nonetheless; certainly no one can argue that graphite, charcoal, and diamond are not different forms of carbon.
References and Resources:
- University of Cincinnati: Ask the Historian – The Origin of the Term Allotrope
- Nuffield Foundation: Allotropes of Sulfur