allotrope
one of at least two different forms of an element. Different allotropes of a substance often have very different properties from each other; e.g.
white phosphorus (
P4) is a highly reactive and toxic substance that glows in air due to its high reactivity and possesses a molecular structure, while red
phosphorus (the stuff used to make strikers on match boxes) is a non-toxic substance that has a polymeric structure.