Allotropy:

Allotropy or allotropism is a behavior exhibited by some chemical elements: these elements can exist in two or more different forms, known as allotropes of that element. In each allotrope, the element's atoms are bonded together in a different manner. Allotropes are different structural modifications of an element. Allotropes should not be confused with isomers, which are chemical compounds that share the same molecular formula but have different structural formulae.

For example, carbon has 3 common allotropes: diamond, where the carbon atoms are bonded together in a tetrahedral lattice arrangement, graphite, where the carbon atoms are bonded together in sheets of a hexagonal lattice, and fullerenes, where the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations.

Allotropy refers only to different forms of an element within the same phase or state of matter (i.e. different solid, liquid or gas forms); the changes of state between solid, liquid and gas in themselves are not considered allotropy. For some elements, allotropes have different molecular formulae which can persist in different phases for example, two allotropes of oxygen (dioxygen, O₂ and ozone, O₃), can both exist in the solid, liquid and gaseous states. Conversely, some elements do not maintain distinct allotropes in different phases for example phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

Differences in properties of an element's Allotropes

Allotropes are different structural forms of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is triggered by the same forces that affect other structures, i.e. pressure, light, and temperature. Therefore the stability of the particular allotropes depends on particular conditions. For instance, iron changes from a body-centered cubic structure (ferrite) to a face-centered cubic structure (austenite) above 906°centigrade, and tin undergoes a transformation known as tin pest from a metallic phase to a semiconductor phase below 13.2°centigrade.