allotropes

Examples of allotropes in the following topics:

• Properties of Sulfur

  • Sulfur burns with blue flame, is insoluble in water, and forms polyatomic allotropes.
  • Sulfur is found is different polyatomic allotropic forms.
  • The best-known allotrope is octasulfur, cyclo-S8.
  • The density of sulfur is about 2 g/cm3, depending on the allotrope.
  • All of sulfur's stable allotropes are excellent electrical insulators.

• Allotropes of Carbon

  • Various allotropes of carbon exhibit different properties and find applications in a variety of fields.
  • There are several allotropes of carbon.
  • Diamond is probably the most well known carbon allotrope.
  • Graphite is another allotrope of carbon; unlike diamond, it is an electrical conductor and a semi-metal.
  • Carbon nanomaterials make up another class of carbon allotropes.

• Properties of Carbon

  • Carbon has several allotropes, or different forms in which it exists.
  • Interestingly, carbon allotropes span a wide range of physical properties: diamond is the hardest naturally occurring substance, and graphite is one of the softest known substances.
  • Allotropes of carbon are not limited to diamond and graphite, but also include buckyballs (fullerenes), amorphous carbon, glassy carbon, carbon nanofoam, nanotubes, and others.
  • Some allotropes of carbon: a) diamond, b) graphite, c) lonsdaleite, d–f) fullerenes (C60, C540, C70); g) amorphous carbon, h) carbon nanotube.

• Properties of Phosphorus

  • Phosphorus is found in its elemental form as different allotropes, none of which are stable in the presence of oxygen.
  • Phosphorus exists in several forms (allotropes) that exhibit strikingly different properties.
  • The two most common allotropes are white phosphorus and red phosphorus.
  • Another allotrope is diphosphorus; it contains a phosphorus dimer as a structural unit and is highly reactive.
  • White phosphorus is the least stable, the most reactive, the most volatile, the least dense, and the most toxic of the allotropes.

• Covalent Crystals

  • Graphite is an allotrope of carbon.
  • In this allotrope, each atom of carbon forms three covalent bonds, leaving one electron in each outer orbital delocalized, creating multiple "free electrons" within each plane of carbon.
  • Diamond is also an allotrope of carbon.
  • These two allotropes of carbon are covalent network solids which differ in the bonding geometry of the carbon atoms.

• Standard States and Standard Enthalpy Changes

  • Standard states for atomic elements are given in terms of the most stable allotrope for each element.
  • For example, white tin and graphite are the most stable allotropes of tin and carbon, respectively.
  • White tin (on the left) is the most stable allotrope of tin, and is used as its standard state for thermodynamic calculations.

• Molecular Crystals

  • For example, solid phosphorus can crystallize in different allotropes called "white", "red" and "black" phosphorus.
  • Although white phosphorus is an insulator, the black allotrope, which consists of layers extending over the whole crystal, does conduct electricity.
  • The structural transitions in phosphorus are reversible: upon releasing high pressure, black phosphorus gradually converts into the red allotrope, and by vaporizing red phosphorus at 490 °C in an inert atmosphere and condensing the vapor, covalent red phosphorus can be transformed back into the white molecular solid.
  • However, they can convert into covalent allotropes having atomic chains extending all through the crystal.

• Fused Benzen Ring Compounds

  • Such a carbon allotrope exists and is called graphite.
  • Another well-characterized carbon allotrope is diamond.
  • These materials represent a third class of carbon allotropes.

• Ozone

  • It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2), breaking down with a half life of about half an hour in the lower atmosphere to O2.

• Elemental Boron

  • Several allotropes of boron exist.