Editing Chemical formula (section)

== Trapped atoms ==
[[File:Endohedral fullerene.png|thumb|180px|Traditional formula: {{chem2|MC60}}<br>The "@" notation: {{chem2|M@C60}}]]
{{main|Endohedral fullerene}}
The @ symbol ([[at sign]]) indicates an atom or molecule trapped inside a cage but not chemically bound to it. For example, a [[buckminsterfullerene]] ({{chem2|C60}}) with an atom (M) would simply be represented as {{chem2|MC60}} regardless of whether M was inside the fullerene without chemical bonding or outside, bound to one of the carbon atoms. Using the @ symbol, this would be denoted {{chem2|M@C60}} if M was inside the carbon network. A non-fullerene example is {{chem2|[As@Ni12As20](3-)}}, an ion in which one [[arsenic]] (As) atom is trapped in a cage formed by the other 32 atoms.

This notation was proposed in 1991<ref name=YanChai>{{cite journal |title=Fullerenes wlth Metals Inside |author1=Chai, Yan |author2=Guo, Ting |author3=Jin, Changming |author4=Haufler, Robert E. |author5=Chibante, L. P. Felipe |author6=Fure, Jan |author7=Wang, Lihong |author8=Alford, J. Michael |author9=Smalley, Richard E. |journal=Journal of Physical Chemistry |volume=95 |issue=20 |year=1991 |pages=7564–7568 |doi=10.1021/j100173a002}}</ref> with the discovery of [[fullerene]] cages ([[endohedral fullerene]]s), which can trap atoms such as [[Lanthanum|La]] to form, for example, {{chem2|La@C60}} or {{chem2|La@C82}}. The choice of the symbol has been explained by the authors as being concise, readily printed and transmitted electronically (the at sign is included in [[ASCII]], which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene.