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3.4: Formulas and names of ionic compounds

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    Learning Objectives
    • Derive names for common types of ionic compounds using a systematic approach

    Nomenclature, a collection of rules for naming things, is important in science and in many other situations. This section describes an approach that is used to name simple ionic such as NaCl, and CaCO3. The simplest of these are binary compounds, those containing only two elements, but we will also consider how to name ionic compounds containing polyatomic ions. We will follow the nomenclature guidelines proposed by IUPAC.

    To name an inorganic compound, we need to consider the answers to several questions. First, is the compound ionic or molecular? If the compound is ionic, does the metal form ions of only one type (fixed charge) or more than one type (variable charge)? Are the ions monatomic or polyatomic?

    Compounds Containing Only Monatomic Ions

    The name of a binary compound containing monatomic ions consists of the name of the cation (the name of the metal) followed by the name of the anion (the name of the nonmetallic element with its ending replaced by the suffix –ide). Some examples are given in Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\): Names of Some Ionic Compounds
    NaCl, sodium chloride Na2O, sodium oxide
    KBr, potassium bromide CdS, cadmium sulfide
    CaI2, calcium iodide Mg3N2, magnesium nitride
    CsF, cesium fluoride Ca3P2, calcium phosphide
    LiCl, lithium chloride Al4C3, aluminum carbide

    Compounds Containing Polyatomic Ions

    Compounds containing polyatomic ions are named similarly to those containing only monatomic ions, except there is no need to change to an –ide ending, since the suffix is already present in the name of the anion. Examples are shown in Table \(\PageIndex{2}\).

    Table \(\PageIndex{2}\): Names of Some Polyatomic Ionic Compounds
    KC2H3O2, potassium acetate (NH4)Cl, ammonium chloride
    NaHCO3, sodium bicarbonate CaSO4, calcium sulfate
    Al2(CO3)3, aluminum carbonate Mg3(PO4)2, magnesium phosphate
    Ionic Compounds in Your Cabinets

    Every day you encounter and use a large number of ionic compounds. Some of these compounds, where they are found, and what they are used for are listed in Table. Look at the label or ingredients list on the various products that you use during the next few days, and see if you run into any of those in this table, or find other ionic compounds that you could now name or write as a formula.

    Ionic Compound Use
    NaCl, sodium chloride ordinary table salt
    KI, potassium iodide added to “iodized” salt for thyroid health
    NaF, sodium fluoride ingredient in toothpaste
    NaHCO3, sodium bicarbonate baking soda; used in cooking (and as antacid)
    Na2CO3, sodium carbonate washing soda; used in cleaning agents
    NaOCl, sodium hypochlorite active ingredient in household bleach
    CaCO3 calcium carbonate ingredient in antacids
    Mg(OH)2, magnesium hydroxide ingredient in antacids
    Al(OH)3, aluminum hydroxide ingredient in antacids
    NaOH, sodium hydroxide lye; used as drain cleaner
    K3PO4, potassium phosphate food additive (many purposes)
    MgSO4, magnesium sulfate added to purified water
    Na2HPO4, sodium hydrogen phosphate anti-caking agent; used in powdered products
    Na2SO3, sodium sulfite preservative

    Compounds Containing a Metal Ion with a Variable Charge

    Most of the transition metals can form two or more cations with different charges. Compounds of these metals with nonmetals are named with the same method as compounds in the first category, except the charge of the metal ion is specified by a Roman numeral in parentheses after the name of the metal. The charge of the metal ion is determined from the formula of the compound and the charge of the anion. For example, consider binary ionic compounds of iron and chlorine. Iron typically exhibits a charge of either 2+ or 3+ (see [link]), and the two corresponding compound formulas are FeCl2 and FeCl3. The simplest name, “iron chloride,” will, in this case, be ambiguous, as it does not distinguish between these two compounds. In cases like this, the charge of the metal ion is included as a Roman numeral in parentheses immediately following the metal name. These two compounds are then unambiguously named iron(II) chloride and iron(III) chloride, respectively. Other examples are provided in Table \(\PageIndex{3}\).

    Table \(\PageIndex{3}\): Names of Some Transition Metal Ionic Compounds
    Transition Metal Ionic Compound Name
    FeCl3 iron(III) chloride
    Hg2O mercury(I) oxide
    HgO mercury(II) oxide
    Cu3(PO4)2 copper(II) phosphate

    Out-of-date nomenclature used the suffixes –ic and –ous to designate metals with higher and lower charges, respectively: Iron(III) chloride, FeCl3, was previously called ferric chloride, and iron(II) chloride, FeCl2, was known as ferrous chloride. Though this naming convention has been largely abandoned by the scientific community, it remains in use by some segments of industry. For example, you may see the words stannous fluoride on a tube of toothpaste. This represents the formula SnF2, which is more properly named tin(II) fluoride. The other fluoride of tin is SnF4, which was previously called stannic fluoride but is now named tin(IV) fluoride.

    Naming Ionic Compounds

    Name the following ionic compounds, which contain a metal that can have more than one ionic charge:

    1. Fe2S3
    2. CuSe
    3. GaN
    4. CrCl3
    5. Ti2(SO4)3


    The anions in these compounds have a fixed negative charge (S2−, Se2, N3−, Cl, and \(\ce{SO4^2-}\)), and the compounds must be neutral. Because the total number of positive charges in each compound must equal the total number of negative charges, the positive ions must be Fe3+, Cu2+, Ga3+, Cr4+, and Ti3+. These charges are used in the names of the metal ions:

    1. iron(III) sulfide
    2. copper(II) selenide
    3. gallium(III) nitride
    4. chromium(III) chloride
    5. titanium(III) sulfate
    Exercise \(\PageIndex{1}\)

    Write the formulas of the following ionic compounds:

    1. chromium(III) phosphide
    2. mercury(II) sulfide
    3. manganese(II) phosphate
    4. copper(I) oxide
    5. chromium(VI) fluoride
    6. Answer


    Chemists use nomenclature rules to clearly name compounds. Ionic and molecular compounds are named using somewhat-different methods. Binary ionic compounds typically consist of a metal and a nonmetal. The name of the metal is written first, followed by the name of the nonmetal with its ending changed to –ide. For example, K2O is called potassium oxide. If the metal can form ions with different charges, a Roman numeral in parentheses follows the name of the metal to specify its charge. Thus, FeCl2 is iron(II) chloride and FeCl3 is iron(III) chloride. Some compounds contain polyatomic ions; the names of common polyatomic ions should be memorized.

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