Page 67 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

F O R M U L A E IR-4.4

14. ([CuCl 3 ] ) n , o r ( [CuCl 3 ]) n , o r
n
n−
Cl
Cu Cl
Cl
n
IR-4.4 S E Q U E N C E O F C I T A T I O N O F S Y M B O L S I N F O R M U L A E

IR-4.4.1 Introduction
Atomic symbols in formulae may be ordered in various ways. Section IR-4.4.3 describes the
conventions usually adopted for some important classes of compounds. A s a prerequisite,
Section IR-4.4.2 explains what is meant by the two ordering p rinciples ‘ electronegativity’
and ‘alphabetical ordering’.

IR-4.4.2 Ordering principles

IR-4.4.2.1 Electronegativity

If electronegativity is taken as the ordering p rinciple in a f ormula o r a part of a f ormula, the
atomic symbols are cited according to relative electronegativities, the least electronegative
element being cited first. For this purpose, Table VI* is used as a g uide. By convention, the
later an element occurs w hen t he table is traversed following the arrows, the more
electropositive is the element.

IR-4.4.2.2 Alphanumerical order
Atomic symbols within line formulae are ordered alphabetically. A s ingle-letter symbol
always precedes a t wo-letter s ymbol with the same initial letter, e.g. B b efore Be, a nd two-
letter symbols are themselves ordered alphabetically, e.g. Ba before B e.
Line formulae for different species can be ordered alphanumerically, e.g. in indexes and
registries, according to the order of the atomic symbols and the right subscripts to these,
e.g. B 5 BH 5 BO 5 B 2 O 3 . T he group NH 4 is often treated as a s ingle symbol and so listed
after Na, for example.
To exemplify, the order of citation o f s ome n itrogen- and sodium-containing entities is:

2

3
2

N ; NH 2 ; NH 3 ; NO 2 ; NO 2 ; NO 3 ; N 2 O 2 ; N 3 ; Na; NaCl; NH 4 Cl

Such ordering m ay be applied to entire formulae in indexes and registries etc., b ut may also be
used for ordering parts of a g iven formula, sometimes in connection with the ordering p rinciple
of Section IR-4.4.2.1, as decribed below for various specific c lasses of compounds and ions.
IR-4.4.3. Formulae for specific c lasses of compounds
IR-4.4.3.1 Binary s pecies
In accordance with established practice, the electronegativity criterion (Section IR-4.4.2.1)
is most often used in binary species. 2
* T ables numbered with a R oman numeral are collected together at the end of this book.

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