What Is an Allotrope? Definition and Examples in Chemistry


Graphite and diamond are two allotropes of carbon.
Graphite and diamond are two allotropes of carbon. Other carbon allotropes include fullerenes, graphene, diamane, glassy carbon, and nanotubules.

Allotropes are defined as different structural forms of a single chemical element. These forms result from the different ways atoms can bond to one another.

Swedish chemist Jöns Jakob Berzelius proposed the concept of allotropy in 1841. The word “allotropy” comes from the Greek word allotropia, which means “changeableness.”

What Allotropes Are and How They Form

Elements transform from one allotrope to another in response to changes in temperature, pressure, and even exposure to light. Allotropes often form spontaneously. Usually, the first solid allotrope to crystallize from a solution or melt is the least stable one. This phenomenon is called Ostwald’s rule or Ostwald’s step rule.

Allotropes have different physical and chemical properties from one another. For example, diamond and graphite (two allotropes of carbon) have different appearances, hardness values, melting points, boiling points, and reactivities.

Some element allotropes have different molecular formulae. Form example, dioxygen (O2) and ozone (O3) exist as separate allotropes in solid, liquid, and gas phases. Some elements have multiple allotropes in the solid phase, but one liquid and gas form. Others have liquid and gas allotropes.

Examples of Allotropes

Most (possibly all) elements have allotropes. The elements with the most allotropes are those with multiple oxidation states. Allotropes of nonmetals are among the most widely recognized, because nonmetals tend to display colors. But, metalloids and metals form allotropes, too.

Here are some examples of allotropes of different elements. Keep in mind, researchers are always discovering new allotropes, particularly those formed under high pressures.

Carbon Allotropes

  • Diamond – tetrahedral lattice
  • Graphite – sheets of hexagonal lattices
  • Graphene – two-dimensional honeycomb lattice
  • Amorphous carbon – non-crystalline
  • Lonsdaleite or hexagonal diamond
  • Fullerenes
  • Nanotubules

Phosphorus Allotropes

  • White phosphorus – crystalline tetraphosphorus (P4)
  • Red phosphorus
  • Violet phosphorus – monoclinic crystals
  • Scarlet phosphorus
  • Black phosphorus
  • Diphosphorus – gaseous P2

Oxygen Allotropes

  • Dioxygen (O2) – colorless gas, pale blue liquid and solid
  • Ozone (O3) – pale blue gas, blue liquid and solid
  • Tetraoxygen (O4) – pale blue to pink
  • Octaoxygen (O8) – red crystals
  • δ-phase – orange
  • ε-phase – black
  • Metallic – forms at extremely high pressures

Arsenic Allotropes

  • Yellow arsenic – molecular non-metallic As4
  • Gray arsenic – polymeric As (metalloid)
  • Black arsenic – molecular and non-metallic

Tin Allotropes

  • α-tin or gray tin – also called tin pest; diamond cubic crystals
  • β-tin or white tin
  • γ-tin – body-centered tetragonal crystals
  • σ-Sn – body-centered cubic crystals

Iron Allotropes

  • α-Fe or ferrite – body-centered cubic
  • γ-iron or austenine – face-centered cubic
  • δ-iron – body-centered cubic
  • ε-iron or hexaferrum – hexagonal close-packed

Allotropism vs Polymorphism

Allotropism refers to different forms of pure chemical elements. Polymorphism refers to different shapes of molecules. Packing polymorphism is when molecules display different crystal structures. Conformational polymorphism refers to different conformers of the same molecule, including isomerization.

Polymorphism is common in binary metal oxides, such as CrO2, Fe2O3, and Al2O3. The different forms are called phases and usually have Greek letters to distinguish them. For example, CrO2 has a tetragonal α phase and an orthorhombic β phase.

Polymorphism is common in pharmaceuticals. Often, solubility and therapeutic effectiveness are very different for polymorphs, so regulatory approval tends to be for a single form.

Two of the allotropes of oxygen, for O2 and O3, were among the first to be recognized. Ostwald considered allotropy to be a special case of polymorphism. But, most chemists refer to different element forms as allotropes and different molecule forms as polymorphs. Technically, molecular oxygen (O2) and ozone (O3) are both allotropes and polymorphs.

References

  • IUPAC (1997). “Allotrope”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). doi:10.1351/goldbook.A00243
  • Jensen, W. B. (2006). “The Origin of the Term Allotrope”. J. Chem. Educ. 83 (6): 838–39. doi:10.1021/ed083p838
  • Threlfall, T. (2003). “Structural and thermodynamic explanations of Ostwald’s Rule”. Organic Process Research & Development. 7 (6): 1017–1027. doi:10.1021/op030026l

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