Allotropy

When a solid element can exist in more than one crystalline form, each with its own physical properties such as colour, density and melting point, the phenomenon is called allotropy, and the individual forms are called allotropes. Several elements of groups 13, 14 and 15 — boron, bismuth and silicon among them — show allotropy, but this chapter focuses on the best-known and most instructive cases: the allotropes of carbon (diamond, graphite, fullerenes, carbon nanotubes and graphene, sec …