ALLOTROPY was the term applied by Berzelius (1841) to the existence of a substance in two or more distinct forms. The modern tendency is to restrict the application of the term to crystalline forms of elements which usually have different physi cal but similar chemical properties, and to use the word "poly morphism" to describe the same property in compounds.
The first instance of polymorphism was noted by Mitscherlich, who in 1812 found that disodium hydrogen phosphate, I existed in two crystalline forms ; this was followed in 1823 by the discovery of the rhombic and monoclinic forms of sulphur (see below). These modifications of sulphur are now said to be "allotropes" or forms. Among inorganic compounds which exhibit allotropy, in the older and broader sense, mention may be made of mercuric iodide ; when this is precipitated by the cautious addition of a solution of potassium iodide to one of mercuric chloride, it may often be seen in a yellow form which changes almost instantly to the more stable red modification. Many organic compounds exhibit dimorphism, that is, there are two crystalline forms of the same chemical individual ; for ex ample, when para-bromoacetanilide first crystallizes from solu tion it does so in needles which are so felted together that they enclose the whole of the liquid and form an apparently solid mass; on standing in contact with the mother-liquor, however, they gradually change to small compact crystals, leaving the liquid quite clear. Both these cases are examples of W. Ostwald's generalization that the less stable form is produced first.
Before the various instances of allotropy are discussed in detail, some general aspects of the phenomenon must be considered. Three types of allotropy are recognized : (I) enantiotropic ; (2) monotropic ; and (3) dynamic.
(I) If each of two varieties involved passes reversibly into the other, at a definite temperature and pressure, and therefore exists only within a certain range of conditions, the allotropy is said to be enantiotropic. Thus, tin exists in the familiar white modification and also in a "grey" form. Above the former is the stable modification, but below that temperature it tends to pass into the latter, since this is the stable form at lower temper atures. Conversely, if grey tin is gradually warmed from o°C. it will tend to become converted into its white allotrope at 2o°, which is therefore said to be the transition temperature.
(2) Monotropic allotropy occurs where the change can only be made to take place in one direction. Thus, antimony can be obtained in an "explosive" variety, which will readily pass into the ordinary stable form with the evolution of much heat, for instance, by warmth, friction or shock; but the latter form cannot be reconverted by any change of temperature or pressure into the unstable modification. This type is irreversible (cf. type 1) .
(3) In dynamic allotropy, two forms may co-exist in a definite proportion which varies with the temperature and pressure. Alex. Smits (1903) studied the first known case of this, namely, that of sulphur. This type of allotropy has the same characteristics as dynamic isomerism (q.v.) or tautomerism, but it relates to elements instead of to compounds.
Since 1911 the whole question of allotropy has been studied in a new light, chiefly owing to the work of A. Smits. According to this author, the underlying cause is to be found in polymerisation, i.e., that different types of molecule contain different numbers of atoms; moreover, not only is a fused element an equilibrium mixture of two or more types of molecule, but also the solid forms separating from the molten liquid are solid solutions of the different types of molecule. Further, this "inner equilibrium" may be established either so rapidly as to make the two molecular species simulate the behaviour of a single type or so slowly that the mixture behaves as if two different individuals are present.
Another general aspect of allotropy must be considered, for any two allotropic forms of an element have different energy contents (except at their transition point) ; the less stable, having the higher content, will evolve heat on changing into the more stable (as in the case of antimony, above) ; also it has a higher vapour pressure and is more soluble in any given solvent (see PHOSPHORUS). Moreover, this energy difference can be made to appear in an electrical form instead of a thermal form, and an e.m.f. can be developed between the two : if electrodes of grey and white tin are immersed in a conducting solution of a tin salt, according as the temperature is above or below 2o° the grey or the white will be unstable and tend to go into solution and be deposited in the other (stable) form—at 2o° (the tran sition temperature) there is no interconversion and no e.m.f. is developed.
Further, the conditions of temperature and pressure tend to influence the interconversion of allotropes according to H. Le Chatelier's principle of "mobile equilibrium," that is, the sub stance tends to assume that form which makes it adapt itself to the external circumstances. Thus increase of pressure favours the production of the denser form, and increase of temperature favours a change which takes place with absorption of heat, and vice versa.
Usually, the introduction of a trace of the form which is stable under the prevailing conditions will bring about the conversion of a supercooled liquid or metastable solid to the stable form.
Cohen has shown that most metals and alloys are in a meta stable state when first formed, and as this may change slowly into a more stable state, the physical properties are somewhat variable. Aluminium, bismuth, cadmium, copper, lead, zinc, and even so dium show evidence of such changes. Mercury shows no change. For the more interesting cases of allotropy see CARBON ; OXYGEN ;