Allotropy
Allotropy is the phenomenon of the formation of elements of two or more simple substances. So simple substance O2 oxygen and a simple substance ozone O3 are allotropic modifications of an element of oxygen. Carbon, phosphorus, sulfur and other elements also form alotropic alterations. Allotropic modifications, or modifications, are determined either by different number of atoms in the molecules, or by different crystalline structure of solids, consisting of atoms of the same chemical element. The phenomena of allotropy is explained by the fact that simple substances are known much more than chemical elements. We know more than 400 simple substances, and chemical elements - only 118.