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3.11: Ionic Bonding: Symbolizing and Naming Polyatomic Ions

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  • Learning Objectives
    • Symbolize and name polyatomic ions.

    The previous sections of this chapter have presented and applied the processes for determining the chemical formula and the chemical name of an ionic compound that contains exclusively main group elements or a combination of main group and transition metal elements.  The ions generated by both main group and transition metal elements are classified as monatomic, or single-atom, ions.  Ionic compounds can also be formed using polyatomic, or multi-atom, ions, which generally bond and react as indivisible charged units.  Polyatomic ions have defined formulas, names, and charges that cannot be modified in any way.  Table \(\PageIndex{1}\) lists the ion names and ion formulas of several common polyatomic ions.  Note that only one polyatomic ion in this Table, the ammonium ion, is a cation.  This polyatomic ion contains one nitrogen and four hydrogens that collectively bear a +1 charge. 

    The remaining polyatomic ions are all negatively-charged and, therefore, are classified as anions.  However, only two of these, the hydroxide ion and the cyanide ion, are named using the "-ide" suffix that is typically indicative of negatively-charged particles.  The remaining polyatomic anions, which all contain oxygen, in combination with another non-metal, exist as part of a series in which the number of oxygens within the polyatomic unit can vary.  As has been repeatedly emphasized in several sections of this text, no two chemical formulas should share a common chemical name.  A single suffix, "-ide," is insufficient for distinguishing the names of the anions in a related polyatomic series.  Therefore, "-ate" and "-ite" suffixes are employed, in order to denote that the corresponding polyatomic ions are part of a series.  Additionally, these suffixes also indicate the relative number of oxygens that are contained within the polyatomic ions.  Note that all of the polyatomic ions whose names end in "-ate" contain one more oxygen than those polyatomic anions whose names end in "-ite."  Unfortunately, much like the common system for naming transition metals, these suffixes only indicate the relative number of oxygens that are contained within the polyatomic ions.  For example, the nitrate ion, which is symbolized as NO3−1, has one more oxygen than the nitrite ion, which is symbolized as NO2−1.  However, the sulfate ion is symbolized as SO4−2.  While both the nitrate ion and the sulfate ion share an "-ate" suffix, the former contains three oxygens, but the latter contains four.  Additionally, both the nitrate ion and the sulfite ion contain three oxygens, but these polyatomic ions do not share a common suffix.  Unfortunately, the relative nature of these suffixes mandates that the ion formula/ion name combinations of the polyatomic ions must simply be memorized.

    Table \(\PageIndex{1}\): Ion Names and Ion Formulas of Common Polyatomic Ions
    Ion Name Ion Formula
    ammonium ion NH4+1
    hydroxide ion OH−1
    cyanide ion CN−1
    carbonate ion CO3−2
    acetate ion C2H3O2−1 or CH3CO2−1
    nitrate ion NO3−1
    nitrite ion NO2−1
    sulfate ion SO4−2
    sulfite ion SO3−2
    phosphate ion PO4−3
    phosphite ion PO3−3