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IR-3.4                                                        E L E M  E N T S





           IR-3.4.2    Allotropes  ( allotropic  m  odifications) of elements
                       Allotropic modifications of an element bear the name  o f  t he atom from which they are
                       derived, together  w  ith a  d escriptor to specify the modification. Common descriptors  a re
                       Greek letters (a, b, g, etc.), colours and, where appropriate, mineral names (e.g. g raphite and
                       diamond for the well known forms of carbon). Such names should be regarded as
                       provisional, to be used only until structures  h ave been established, after which a  r ational
                       system based on molecular formula (see Section IR-3.4.3) or crystal structure (see Section
                       IR-3.4.4) is recommended. Common names will continue to be used for amorphous
                       modifications of an element a nd for those which are mixtures of closely related structures in
                       their commonly occurring forms (such  a  s  g  raphite) or have an ill-defined disordered
                       structure (such as red phosphorus) (see Section IR-3.4.5).

           IR-3.4.3    Names of allotropes of definite molecular formula

                       Systematic names are based on the number of atoms in the molecule, indicated by a
                       multiplicative prefix from Table IV. The  p refix ‘mono’ is only used when the element does
                       not normally occur in a  m  onoatomic state. If the number is large and unknown, as in long
                       chains or large rings, the prefix ‘poly’ may be used. Where necessary, appropriate prefixes
                       (Table V) may be used to indicate structure.  W  hen it is desired to specify a  p articular
                       polymorph of an element with a  d efined structure (such  a s  t he a-, b- o r g-forms of S 8 ) t he
                       method of Section IR-3.4.4 should be used (see Examples  1 3–15 in Section IR-3.4.4).
                       Examples:

                                 Formula          Systematic name       Acceptable  a lternative name
                           1.       Ar            argon
                           2.       H             m  onohydrogen
                           3.       N             m  ononitrogen
                           4.       N 2           dinitrogen
                           5.       N 3 *         trinitrogen( * )
                           6.       O 2           dioxygen              oxygen
                           7.       O 3           trioxygen             ozone
                           8.       P 4           tetraphosphorus       white phosphorus
                           9.       S 6           hexasulfur            e-sulfur
                          10.       S 8           cyclo-octasulfur      a-sulfur, b-sulfur, g-sulfur
                          11.       S n           polysulfur            m-sulfur (or plastic sulfur)
                          12.       C 60          hexacontacarbon       [60]fullerene


                       In Example 12, the name  [ 60]fullerene is to be regarded as an acceptable non-systematic
                       name for a  p articular C 60 structure. For more  d etails see Section P-27 of Ref. 6.


           IR-3.4.4    Crystalline allotropic modifications of an element
                       Crystalline allotropic  m  odifications are polymorphs of the elements. Each can be named by
                                                                 7
                       adding the Pearson symbol (see Section IR-11.5.2) in parentheses after the name of the

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