Category I and II Ionic Compounds Formed with Polyatomic
Ions
Another group of ionic compounds contains polyatomic ions. A polyatomic ion is
a group of elements that act like a single ion when forming a compound. The
bonds within these polyatomic ions are predominantly covalent. However, the
group as a whole has an excess charge, which is usually negative, because of an
excess of electrons. If the compounds formed with the polyatomic ions consist of
three elements, they are called ternary compounds.
Polyatomic ions have special names and formulas that you must memorize.
Table 6 contains the names and ionic charges of the common polyatomic ions
encountered in a first-year chemistry course. Note that only one commonly used
positively charged polyatomic ion is in Table 6, the ammonium ion, NH 4 +.
Also notice in Table 6 that several of the polyatomic anions contain an atom
of a given element and a different number of oxygen atoms, such as NO 2 and NO 3.
When there are two members of such a series, the name of the one with fewer
oxygen atoms ends in -ite and the name of the one with more oxygen atoms ends in
–ate. The following table shows examples of polyatomic ions of sulfur.