one of at the least two different forms of an element. Different allotropes of a substance frequently have different properties from both; e.g. white phosphorus (P4) is a highly reactive and poisonous compound that glows in atmosphere because of its large reactivity and possesses a molecular structure, while red phosphorus (the stuff accustomed make strikers on match boxes) is a non-toxic material which have a polymeric framework.
1847, back-formation from allotropy "variation of physical properties without change of compound," from allo- + -tropy "manner" (see -trope). Relevant: Allotropic.
a structurally different form of an element
Hittorf's phosphorus is another crystalline allotrope formed by heating phosphorus with lead in a sealed tube to redness, and removing the lead by boiling the product with nitric and hydrochloric acid.