However, the shape of the ion is described by the locations of the atoms, not
the lone pair. Because only three of the tetrahedral locations are occupied by atoms,
the shape of an SO3
2 ion is trigonal pyramidal (17). The rule to remember is
Rule 3 All regions of high electron density, lone pairs and bonds, are included
in a description of the electronic arrangement, but only the positions of atoms
are considered when identifying the shape of a molecule.
A single unpaired electron on the central atom also is a region of high electron
density and is treated like a lone pair when determining molecular shape. For example,
radicals such as NO2 have a single nonbonding electron, a “lone half-pair.”
Thus, NO2 (18) has a trigonal planar electron arrangement (including the unpaired
electron on N), but its shape, the arrangement of its atoms, is angular (19).