<P> A left - hand subscript is sometimes used redundantly to indicate the atomic number . For example, O for dioxygen, and O for the most abundant isotopic species of dioxygen . This is convenient when writing equations for nuclear reactions, in order to show the balance of charge more clearly . </P> <P> The @ symbol (at sign) indicates an atom or molecule trapped inside a cage but not chemically bound to it . For example, a buckminsterfullerene (C) with an atom (M) would simply be represented as MC 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 M@C if M was inside the carbon network . A non-fullerene example is (As@Ni As), an ion in which one As atom is trapped in a cage formed by the other 32 atoms . </P> <P> This notation was proposed in 1991 with the discovery of fullerene cages (endohedral fullerenes), which can trap atoms such as La to form, for example, La@C or La@C . 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 . </P> <P> Chemical formulas most often use integers for each element . However, there is a class of compounds, called non-stoichiometric compounds, that cannot be represented by small integers . Such a formula might be written using decimal fractions, as in Fe O, or it might include a variable part represented by a letter, as in Fe O, where x is normally much less than 1 . </P>

A multiple of an empirical formula is often called a