Finally, the same general procedure is used to determine the Lewis structures of
polyatomic ions, except that we add or subtract electrons to account for the charge
on the ion. We count the electrons available for bonds and lone pairs, arrange the
atoms in the appropriate order, and then construct the Lewis structure. As for neutral
molecules, it is essential to know the general arrangement of atoms in the ion.
For oxoanions, it is usually the case that (except for H) the atom written first in the
chemical formula is the central atom. In CO3
2, for instance, the C atom is surrounded
by three O atoms. Each atom provides the number of dots (electrons)
equal to the number of electrons in its valence shell, but we have to adjust the total
number of dots to represent the overall charge. For a cation, we subtract one dot
for each positive charge. For an anion, we add one dot for each negative charge.
The cation and the anion must be treated separately: they are individual ions and
are not linked by shared pairs. The Lewis structure of ammonium carbonate,
(NH4)2CO3, for instance, is written as three bracketed ions (6)