The phenomenon of existence of a chemical element in two or more forms differing in physical properties but having almost same chemical nature is known as allotropy. If an element or compound exists in two or more forms, it is also known as polymorphism e.g. zinc blende and wurtzite are polymorphs of ZnS.

Kinds of allotropy. Allotropy is of three types :

(i) Enantiotropy : When two forms of a solid substance exist together in equilibrium with each other at a particular temperature under normal pressure it is called enantiotropy.

For example, at normal pressure and temperature between 368.6K and 285K , sulphur (solid) exist in two forms (rhombic sulphur),S_R and (monoclinic sulphur), S_M in equilibrium with each other. S_R S_M

(ii) Monotropy: It is the type of allotropy in which only one allotrope is stable, under normal conditions the other being unstable e.g., diamond and graphite, oxygen and ozone etc.

(iii) Dynamictropy: It is the type of allotropy in which there is a true equilibrium between the two allotropes, one changing into the other at exactly the same rate as the reverse occurs. Both allotropes are stable over a wide range of temperature.

For example, liquid sulphur exist in two forms, the pale yellow mobile form called S_λ and dark viscous form called S_µ in equilibrium with each other. S_λ S_µ

With increase in temperature, the later form is formed at the expense of the former but when the liquid is cooled, the reverse change occur. Thus sulphur shows both enantiotropy and dynamictropy.

Cause of allotropy : (i) In general the allotropy among solid substances is due to the difference in crystalline structure. (ii) It may also be due to the presence of different number of atoms e.g., O₂ and O₃, S₈, S₂ etc. (iii) It may be due to the difference in nuclear spins e.g., ortho and para hydrogen.

Different allotropic forms Except lead, all elements show allotropy.

(i) Carbon has two crystalline allotropic forms i.e., diamond and graphite. In diamond C atom is SP³ hybridised and it has a three dimensional network structure. Since no valence electron is available, hence diamond is a bad conductor of electricity. However in graphite C atom is SP² hybridised and has a delocalised π-electron cloud responsible for its high electrical conductivity. It may be noted that diamond is thermodynamically less stable than graphite at ordinary temperatures.

(ii) Silicon has both crystalline and amorphous forms.

(iii) Tin has three crystalline modifications with the following equilibrium temperature

(Grey) (White) (Rhombic)

The conversion of white tin to grey tin is accompanied by an increase in volume and the latter, being very brittle, easily crumbles down to powder. This phenomenon is called tin disease tin pest or tin plague.