Allotropy
Allotropy or allotropism (from Ancient Greek ἄλλος (allos) 'other' and τρόπος (tropos) 'manner, form') is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: the atoms of the element are bonded together in different manners. For example, the allotropes of carbon include diamond (the carbon atoms are bonded together to form a cubic lattice of tetrahedra), graphite (the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (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 compound, is polymorphism, although its use is usually restricted to solid materials such as crystals. Allotropy refers only to different forms of an element within the same physical phase (the state of matter, i.e. plasmas, gases, liquids, or solids). The differences between these states of matter would not alone constitute examples of allotropy. Allotropes of chemical elements are frequently referred to as polymorphs or as phases of the element.
For some elements, allotropes have different molecular formulae or different crystalline structures, as well as a difference in physical phase; for example, two allotropes of oxygen (dioxygen, O2, and ozone, O3) can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different physical phases; for example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.
| Feature | Isomerism (Isomers) | Allotropy (Allotropes) | Polymorphism (Polymorphs) |
|---|---|---|---|
| Core Concept | Molecules with identical molecular formulas but distinct structural arrangements or spatial orientations. | Distinct structural modifications of a single element within the same physical state. | The capacity of a solid compound or element to manifest in multiple distinct crystal lattices. |
| Scope of Applicability | Compounds and molecules (organic and inorganic). | Elements exclusively. | Crystalline solids (elements, ionic salts, or molecular solids). |
| Structural Variation | Variations in chemical bonding, atom connectivity, or three-dimensional spatial geometry. | Variations in bonding patterns, molecular formulas, or crystal structures of an element. | Variations in the packing arrangements or space groups of atoms or molecules in a solid. |
| Chemical Properties | Significant variances in reactivity and functionality (e.g., ethanol versus dimethyl ether). | Marked divergence in chemical reactivity (e.g., the inertness of diamond versus the reactivity of graphite). | Identity in liquid or gaseous phases; minor variations in solid-state surface reactivity. |
| Physical Properties | Variances in boiling points, melting points, and densities. | Radical divergence in characteristics (e.g., the transparency and hardness of diamond versus the opacity and softness of graphite). | Distinct variations in solubility, hardness, density, and melting points. |
| Physical State | Gas, liquid, and solid phases. | Predominantly solid phases, with occurrences in liquid and gas phases (e.g., dioxygen versus ozone gas). | Crystalline solids exclusively. |
| Primary Mechanism | Alternative modes of atomic linkage, functional group placement, or molecular rotation. | Alternative modes of homonuclear bonding and molecular aggregation. | Thermodynamic variations in temperature and pressure during the process of crystallization. |
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