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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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