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5.7: Ionic Compounds Containing Polyatomic Ions

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    ⚙️ Learning Objectives

    • Write correct names and formulas for ionic compounds containing polyatomic ions.


    Some ions consist of groups of atoms bonded together that carry an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, the nitrate ion has one nitrogen atom and three oxygen atoms with an overall charge of 1−. The formula of the nitrate ion is written as NO3. A list of the most common polyatomic ions appears in Table \(\PageIndex{1}\). Since these ions are regularly used throughout the rest of this text, it generally a good practice to memorize this list of ions.
     

    Table \(\PageIndex{1}\): The Most Common Polyatomic Ions
    Name Formula
    ammonium ion NH4+
    acetate ion C2H3O2
    bicarbonate ion (hydrogen carbonate ion) HCO3
    carbonate ion CO32
    chlorate ion ClO3
    cyanide ion CN
    hydroxide ion OH
    nitrate ion NO3
    phosphate ion PO43
    sulfate ion SO42


    Writing Formulas for Ionic Compounds Containing Polyatomic Ions

    Writing formulas for ionic compounds containing polyatomic ions involves the same steps as for a binary ionic compound. The cation is written first in the chemical formula followed by the anion. Consider potassium nitrate. Potassium is located in Group IA of the periodic table, so it forms ions with a 1+ charge, K+. As shown above in Table \(\PageIndex{1}\), the formula of the nitrate ion is NO3. Since the charges of the two ions are already balanced, the formula for potassium nitrate may be written as KNO3.

    When more than one polyatomic ion is required to balance charge, the formula of the polyatomic ion is enclosed in parentheses followed by a subscript that indicates the quantity of that polyatomic ion required to balance the charge. This means that the formula of calcium nitrate is written as Ca(NO3)2, since calcium is located in Group IIA and forms ions with a 2+ charge, Ca2+, and will require two nitrate ions, NO3, to balance the charge of a single calcium ion.

    It should be emphasized that enclosing a polyatomic ion in parentheses is only done when more than one is required to balance charge. Parentheses are not included when only one polyatomic ion is required to balance charge. That is why the formula of potassium nitrate is written as KNO3 and not as K(NO3).
     

    ✅ Example \(\PageIndex{1}\): Writing Chemical Formulas

    Write the chemical formulas for:

    • copper(I) carbonate
    • aluminum acetate
    • magnesium hydroxide
       

    Solution

      copper(I) carbonate aluminum acetate magnesium hydroxide
    1. Write the symbol and charge of the cation and the anion.
    Cu+ CO32 Al3+ C2H3O2 Mg2+ OH
    1. Balance the charge of the cations and anions.
    Cu+
    Cu+
    CO32 Al3+ C2H3O2
    C2H3O2
    C2H3O2
    Mg2+ OH
    OH
    1. Explanation.
    2+ balances 2– 3+ balances 3– 2+ balances 2–
    1. Write the final formula. Charges are left off. Subscripts of 1 are implied.
    Cu2CO3 Al(C2H3O2)3 Mg(OH)2


    Writing Names for Ionic Compounds Containing Polyatomic Ions

    The reverse process is used when writing names for ionic compounds containing polyatomic ions. Each ion is named in the order that they appear in the chemical formula. The most important part is to recognize that a polyatomic ion is present in the compound. Na3PO4 would be called sodium phosphate, since it contains the sodium ion, Na+, and the phosphate ion, PO43. Mg(ClO3)2 would be called magnesium chlorate, since it contains the magnesium ion, Mg2+, and the chlorate ion, ClO3.
     

    ✅ Example \(\PageIndex{2}\): Writing Chemical Names

    Write the name for each compound.

    1. (NH4)2S
    2. FeSO4
    3. Fe2(SO4)3

    Solution

    1. NH4+ is the ammonium ion. S = sulfur. The root name of sulfur is sulf-. The nonmetal has a suffix of -ide. (NH4)2S is ammonium sulfide.
       
    2. SO42 is the sulfate ion.  Fe is a transition element that forms ions with multiple charges. Since only one iron ion is needed to balance the charge of one sulfate ion, it must have a 2+ charge. An iron ion with a 2+ charge is called the iron(II) ion. FeSO4 is iron(II) sulfate.
       
    3. SO42 is the sulfate ion.  Fe is a transition element that forms ions with multiple charges. Since two iron ions are needed to balance the charge of three sulfate ions, it must have a 3+ charge. An iron ion with a 3+ charge is called the iron(III) ion. Fe2(SO4)3 is iron(III) sulfate.


    Oxyanions

    You may have noticed that most of the polyatomic ions in Table \(\PageIndex{1}\) carry a negative charge and contain the element oxygen. Anions that contain the element oxygen are called oxyanions. Many oxyanions belong to a series where the number of oxygen atoms in the ion varies, as shown in Table \(\PageIndex{2}\).
     

    Table \(\PageIndex{2}\): Oxyanions Belonging to a Series
    Name Formula
    chlorate ion ClO3
    chlorite ion ClO2
    nitrate ion NO3
    nitrite ion NO2
    sulfate ion SO42
    sulfite ion SO32


    Note that oxyanions may end with either an -ate or -ite suffix. In other words, if you encounter a compound (or polyatomic ion) that ends with -ate or -ite, it will always contain oxygen. To be clear, simply containing oxygen doesn't mean the compound will always end with an -ate or -ite suffix. We have seen examples of oxides, where oxygen is part of a binary compound such as lithium oxide, Li2O, and hydroxides, where oxygen is part of the polyatomic ion OH.

    When comparing oxyanions that end with -ate and -ite, the ion that ends with -ate will always have one more oxygen than the corresponding ion that ends with -ite. The ion in the series that ends with -ate is the one found in Table \(\PageIndex{1}\): The Most Common Polyatomic Ions. This is because the -ate form is more commonly found in nature than the -ite form. It is also the reason why memorizing the form that ends with -ate is recommended rather than the one that ends with -ite. Furthermore, if you learn the one that ends with -ate, the ion that ends with -ite can easily be obtained, since it always has one less oxygen than the -ate form.

    It should also be pointed out that just because the -ite form always has one less oxygen than the -ate form doesn't mean that an -ite form is possible. Even though the carbonate ion, CO32, is quite common, there is no such thing as the carbonite ion. Some of you reading this might be saying right now, "Wait a minute! I remember hearing about carbonite in Star Wars!" That's true. However, Star Wars is science fiction and may have even been the reason for the choice of using the word carbonite...it doesn't really exist!
     

    ✅ Example \(\PageIndex{3}\)

    1. What is the formula of zinc sulfite?
    2. Write the chemical name of Sr(NO2)2?
    3. If the bromate ion has a formula of BrO3, what is the formula of the bromite ion?

    Solution

    1. From Figure 5.5.1, Zn is one of the three transition elements that has a fixed charge. Zinc ions have a charge of 2+, Zn2+. The formula of the sulfite ion is SO32, which has one less oxygen atom than the sulfate ion. The charge on Zn2+ balances the charge on SO32, so the formula of zinc sulfite is ZnSO3.
           
    2. Sr2+ is the formula of the strontium ion. NO2 is the formula of the nitrite ion, which has one less oxygen than the nitrate ion. Sr(NO2)2 is strontium nitrite.
           
    3. The -ite form always contains one less oxygen atom that the ion with an -ate suffix. If the bromate ion is BrO3, then the bromite ion has a formula of BrO2.

     

    ✏️ Exercise \(\PageIndex{1}\)

    Write the formula for each:

    1. silver cyanide
    2. calcium bicarbonate
    3. lithium chlorite
    4. nickel(III) sulfate
    5. phosphite ion
    Answer A
    AgCN
    Answer B
    Ca(HCO3)2
    Answer C
    LiClO2
    Answer D
    Ni2(SO4)3
    Answer E
    PO33

     

    ✏️ Exercise \(\PageIndex{2}\)

    Write the name of each:

    1. Na2SO3
    2. Cd(C2H3O2)2
    3. (NH4)3PO4
    4. KOH
    5. Pb(NO3)4
    Answer A
    sodium sulfite
    Answer B
    cadmium acetate
    Answer C
    ammonium phosphate
    Answer D
    potassium hydroxide
    Answer E
    lead(IV) nitrate


    Additional Polyatomic Ions

    There are many other polyatomic ions that may be encountered in this text besides the ones already listed. Table \(\PageIndex{3}\) is not an all-inclusive list of additional polyatomic ions that exist, but rather ions you may expect to see in this text. Ions are grouped together if they are a part of a series.
     

    Table \(\PageIndex{3}\): Additional Polyatomic Ions
    Name Formula
    hypochlorite ion ClO
    perchlorate ion ClO4
    chromate ion CrO42
    dichromate ion Cr2O72
    oxalate ion C2O42
    permanganate ion MnO4
    monohydrogen phosphate ion HPO42
    dihydrogen phosphate ion H2PO4
    bisulfate ion (hydrogen sulfate ion) HSO4
    bisulfite ion (hydrogen sulfite ion) HSO3

      

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