Editing Chemical element (section)

=== Allotropes ===
{{Main|Allotropy}}

Atoms of chemically pure elements may bond to each other chemically in more than one way, allowing the pure element to exist in multiple [[chemical structure]]s ([[Molecular geometry|spatial arrangements of atoms]]), known as [[allotrope]]s, which differ in their properties. For example, carbon can be found as [[diamond]], which has a tetrahedral structure around each carbon atom; [[graphite]], which has layers of carbon atoms with a hexagonal structure stacked on top of each other; [[graphene]], which is a single layer of graphite that is very strong; [[fullerene]]s, which have nearly spherical shapes; and [[carbon nanotube]]s, which are tubes with a hexagonal structure (even these may differ from each other in electrical properties). The ability of an element to exist in one of many structural forms is known as 'allotropy'.

The reference state of an element is defined by convention, usually as the thermodynamically most stable allotrope and physical state at a pressure of 1 [[bar (unit)|bar]] and a given temperature (typically 298.15[[Kelvin|K]]). However, for phosphorus, the reference state is white phosphorus even though it is not the most stable allotrope, and the reference state for carbon is graphite, because the structure of graphite is more stable than that of the other allotropes. In [[thermochemistry]], an element is defined to have an [[Standard enthalpy of formation|enthalpy of formation]] of zero in its reference state.