Allotropy

ALLotropy (from Greek allos- other and tropos- form), property of certain chemical elements to exist in several modifications, different in physical and chemical properties. Causes of A.: polymerism (see) - different number of atoms in the molecule (e.g., in O, S, P), different structure of the crystal lattice (in C, Sn) and others. Smits (A. Smits) gave a general theory of A., according to which all substances consist of molecules of varying complexity; in individual allotropic modifications, molecules of a certain complexity predominate. Examples of A.: carbon, known in three solid modifications: diamond Ca crystallizes in the regular system, is transparent, possesses strong luster and refraction, high hardness and specific gravity (3.5); graphite C£ crystallizes in monoclinic plates, is opaque, gray in color, very soft; coal C7-amorphous, black in color. Tin Sn« white in color, lustrous, at temperatures below 18° (transition point) slowly transforms into a gray modification Sn^, brittle and crumbly; in frost the transformation occurs faster ("tin plague"). Sulfur is known in several modifications: rhombic Sa, stable at temperatures below 96.5°; monoclinic, viscous, obtained when heating sulfur above the melting point; plastic, obtained by rapid cooling of molten sulfur, etc. Phosphorus: white-crystalline in the regular system, violet-in monoclinic, red-amorphous; of these, white phosphorus is highly poisonous, while red is harmless. Among gaseous substances, A. is observed, for example, in oxygen O₂ and ozone O₃. A. has also been found in selenium, tellurium, arsenic, boron, silicon and other elements.