- when an element (type of stable atom) exists in two or more forms. These forms may be variations in the way the constituent atoms are arranged into the solids (crystals) or in the number of atoms found in basic molecular arrangement(s).
Monotropic allotropes have one form which is the most stable under all normal conditions.
Enantiotropic allotropes have several forms, each of which is stable under different sets of conditions. It is possible to convert one form into another form by change these conditions (such as temperature and pressure).
The elements carbon, oxygen, sulfur, tin and phosphorus all have allotropic forms. Carbon is considered to be monotropic, as graphite is more stable than diamond. Oxygen is also considered to be monotropic as molecular oxygen (O2 - in the air) is more stable that ozone (O3 - triatomic oxygen) under natural conditions.
Tin is considered to be enantiotropic as gray tin is stable at temperatures below 13o C and converts to the white form of tin at higher temperatures.