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3.5: Ionic Compounds- Formulas and Names

  • Page ID
    37955
  • 6.9: Binary Ionic Compounds and Their Properties

    6.18: Ionic Compounds Containing Polyatomic Ions

    Skills to Develop

    • Derive names for common types of inorganic compounds using a systematic approach

    Nomenclature, a collection of rules for naming things, is important in science and in many other situations. This module describes an approach that is used to name simple ionic and molecular compounds, such as NaCl, CaCO3, and N2O4. 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, and one specific, very important class of compounds known as acids (subsequent chapters in this text will focus on these compounds in great detail). We will limit our attention here to inorganic compounds, compounds that are composed principally of elements other than carbon, and will follow the nomenclature guidelines proposed by IUPAC. The rules for organic compounds, in which carbon is the principle element, will be treated in a later chapter on organic chemistry.

    Ionic Compounds

    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? If the compound is molecular, does it contain hydrogen? If so, does it also contain oxygen? From the answers we derive, we place the compound in an appropriate category and then name it accordingly.

    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 \(\PageIndex{2}\).

    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.

    Example \(\PageIndex{1}\): 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

    Solution

    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. (a) chromium(III) phosphide
    2. (b) mercury(II) sulfide
    3. (c) manganese(II) phosphate
    4. (d) copper(I) oxide
    5. (e) chromium(VI) fluoride

    Answer:

    (a) CrP; (b) HgS; (c) Mn3(PO4)2; (d) Cu2O; (e) CrF6

    Summary

    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. Molecular compounds can form compounds with different ratios of their elements, so prefixes are used to specify the numbers of atoms of each element in a molecule of the compound. Examples include SF6, sulfur hexafluoride, and N2O4, dinitrogen tetroxide. Acids are an important class of compounds containing hydrogen and having special nomenclature rules. Binary acids are named using the prefix hydro-, changing the –ide suffix to –ic, and adding “acid;” HCl is hydrochloric acid. Oxyacids are named by changing the ending of the anion to –ic, and adding “acid;” H2CO3 is carbonic acid.

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