What is an Allotrope?

Allotropy is the ability of a chemical to exhibit a number of different and physically distinct forms in its pure elemental state. Carbon, for instance, can exist as graphite, diamond , and fullerene (including the buckyball, C60). Typically, elements capable of variable coordination numbers and/or oxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element to catenate. Allotropes are typically more noticeable in non-metals and metalloids. The term allotropes may also be used to refer to the molecular forms of an element (such as a diatomic gas), even if there is only one such additional form.

What is an isomer?