44 STRUCTURE AND BONDINGion. for example, contains carbon covalently bonded to three
oxygen atoms and we can write the structure as :Clearly such bonding would produce two different carbon-oxygen
bond distances (p. 48) but in fact all bonds are found to be identical
and intermediate in length between the expected C=O and C — O
bond distances. We conclude, therefore, that the true structure of
the carbonate ion cannot be accurately represented by any one
diagram of the type shown and a number of 'resonance' structures
are suggested (p. 50).O=C ^
-
As in the case of NH^ the charge is distributed over the whole ion.
By considering each multiple bond to behave spatially as a single
bond we are again able to use Table 2.8 to correctly deduce that the
carbonate ion has a trigonal planar symmetry. Structures for other
covalently-bonded ions can readily be deduced.COMPLEX IONS
The polyatomic ions discussed above are really simple members of
a much larger group known collectively as complex ions, in which a
central atom or ion is surrounded by other atoms, ions or groups
of atoms, called ligands. Whenever an ion is formed in a polar
solvent, ion-dipole attraction causes the solvent molecules to
orientate themselves around the ion producing a solvated ion, for
example [Na(H 2 O)J +. For large ions of small charge these attrac-
tive forces are weak and are not of any great importance. However,
the greater the charge on the central ion and the smaller its size, the
greater the force of attraction between the ion and the ligand, and
the more covalent the link between them becomes; as in the case of
simple covalent-ionic bonding (p. 50) there is no sharp dividing
line. Salts of Groups I and II clearly show the changes which
accompany increases in ionic size. For example, for a given anion,
the number of water molecules crystallising in the salt is found to
increase as the size of the ion decreases.