As allotropy ( from Greek: ἀλλοτροπέω - change or ἀλλοτρόπως - in a different way ) refers to the phenomenon when a chemical element in the same physical state in two or more structural forms occur, which differ from each other physically and in their chemical reactivity. Be Allotropien in the chemical referred to as modifications of a chemical element.
- Carbon is present both as colorless, extremely hard diamond before, which is an electrical insulator, as well as a black, soft graphite, which conducts electricity, as well as macromolecules fullerene, carbon nanotubes and graphene as.
- White phosphorus is highly toxic and spontaneously combustible, whereas red and black phosphorus are non-toxic and need to burn an activation.
- The oxygen in the air we breathe (O2, dioxygen ) is odorless and vital, its allotropic variant of ozone (O3 ) pungent odor and is already at very low levels (> 100 ppb ) harmful.
- Tin has the forms α and β -Sn -Sn. The conversion of β -Sn to α -Sn the tin pest can occur.
Further examples are known, for example, the elements sulfur, selenium, and iron ( α - iron, γ - iron, etc. ) dar.
Definition of the term
When different isotopes of an element one does not speak of allotropy, as these do not differ or only extremely low in their chemical properties.
If a chemical compound in the solid state in several crystal forms ( modifications ), it is called polymorphism.
Are two modifications due to temperature or pressure changes mutually interconvertible, one speaks of enantiotropy. If this conversion is only in one direction, while the other form can be represented only indirectly, so this transition is called monotropic (Example: A polymeric phosphorus).