• General College Chemistry
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  • Unit I. Atoms
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    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, K2is 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 (–ate to –ic and –ite to –ous), and adding “acid;” H2CO3 is carbonic acid.

    14.1 Chemical Nomenclature

    Learning Objectives

    By the end of this section, you will be able to:

    • 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 14.1.

    Table 14.1

    Names of Some Ionic Compounds

    NaCl, sodium chlorideNa2O, sodium oxide
    KBr, potassium bromideCdS, cadmium sulfide
    CaI2, calcium iodideMg3N2, magnesium nitride
    CsF, cesium fluorideCa3P2, calcium phosphide
    LiCl, lithium chlorideAl4C3, aluminum carbide

    Compounds Containing Polyatomic Ions

    Compounds containing polyatomic ions are named similarly to those containing only monatomic ions, i.e., by naming first the cation and then the anion. Examples are shown in Table 14.2.

    Table 14.2

    Names of Some Polyatomic Ionic Compounds
    KC2H3O2, potassium acetateNH4Cl, ammonium chloride
    NaHCO3, sodium bicarbonateCaSO4, calcium sulfate
    Al2(CO3)3, aluminum carbonateMg3(PO4)2, magnesium phosphate

    Chemistry in Everyday Life

    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 14.3. 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.

    Table 14.3

    Everyday Ionic Compounds

    Ionic CompoundUse
    NaCl, sodium chlorideordinary table salt
    KI, potassium iodideadded to “iodized” salt for thyroid health
    NaF, sodium fluorideingredient in toothpaste
    NaHCO3, sodium bicarbonatebaking soda; used in cooking (and as antacid)
    Na2CO3, sodium carbonatewashing soda; used in cleaning agents
    NaOCl, sodium hypochloriteactive ingredient in household bleach
    CaCO3 calcium carbonateingredient in antacids
    Mg(OH)2, magnesium hydroxideingredient in antacids
    Al(OH)3, aluminum hydroxideingredient in antacids
    NaOH, sodium hydroxidelye; used as drain cleaner
    K3PO4, potassium phosphatefood additive (many purposes)
    MgSO4, magnesium sulfateadded to purified water
    Na2HPO4, sodium hydrogen phosphateanti-caking agent; used in powdered products
    Na2SO3, sodium sulfitepreservative

    Compounds Containing a Metal Ion with a Variable Charge

    Most of the transition metals and some main group 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 Figure 14.1), 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 14.4.

    Table 14.4

    Some Ionic Compounds with Variably Charged Metal Ions
    FeCl2iron(II) chloride
    FeCl3iron(III) chloride
    Hg2Omercury(I) oxide
    HgOmercury(II) oxide
    SnF2tin(II) fluoride
    SnF4tin(IV) fluoride

    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.

    Ionic Hydrates

    Ionic compounds that contain water molecules as integral components of their crystals are called hydrates. The name for an ionic hydrate is derived by adding a term to the name for the anhydrous (meaning “not hydrated”) compound that indicates the number of water molecules associated with each formula unit of the compound. The added word begins with a Greek prefix denoting the number of water molecules (see Table 14.5) and ends with “hydrate.” For example, the anhydrous compound copper(II) sulfate also exists as a hydrate containing five water molecules and named copper(II) sulfate pentahydrate. Washing soda is the common name for a hydrate of sodium carbonate containing 10 water molecules; the systematic name is sodium carbonate decahydrate.

    Formulas for ionic hydrates are written by appending a vertically centered dot, a coefficient representing the number of water molecules, and the formula for water. The two examples mentioned in the previous paragraph are represented by the formulas

    copper(II) sulfate pentahydrateCuSO45H2Osodium carbonate decahydrateNa2CO310H2Ocopper(II) sulfate pentahydrateCuSO45H2Osodium carbonate decahydrateNa2CO310H2O

    Table 14.5

    Nomenclature Prefixes

    NumberPrefix NumberPrefix
    1 (sometimes omitted)mono- 6hexa-

    Example 14.1

    Naming Ionic Compounds

    Name the following ionic compounds:

    (a) Fe2S3

    (b) CuSe

    (c) GaN

    (d) MgSO4∙7H2O

    (e) Ti2(SO4)3


    The anions in these compounds have a fixed negative charge (S2−, Se2− , N3−, and SO42−),SO42−), 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+, Mg2+, and Ti3+. These charges are used in the names of the metal ions:

    (a) iron(III) sulfide

    (b) copper(II) selenide

    (c) gallium(III) nitride

    (d) magnesium sulfate heptahydrate

    (e) titanium(III) sulfate

    Check Your Learning

    Write the formulas of the following ionic compounds:

    (a) chromium(III) phosphide

    (b) mercury(II) sulfide

    (c) manganese(II) phosphate

    (d) copper(I) oxide

    (e) iron(III) chloride dihydrate

    (a) CrP; (b) HgS; (c) Mn3(PO4)2; (d) Cu2O; (e) FeCl3∙2H2O

    Chemistry in Everyday Life

    Erin Brokovich and Chromium Contamination

    In the early 1990s, legal file clerk Erin Brockovich (Figure 14.2) discovered a high rate of serious illnesses in the small town of Hinckley, California. Her investigation eventually linked the illnesses to groundwater contaminated by Cr(VI) used by Pacific Gas & Electric (PG&E) to fight corrosion in a nearby natural gas pipeline. As dramatized in the film Erin Brokovich (for which Julia Roberts won an Oscar), Erin and lawyer Edward Masry sued PG&E for contaminating the water near Hinckley in 1993. The settlement they won in 1996—$333 million—was the largest amount ever awarded for a direct-action lawsuit in the US at that time.

    Figure 14.2

    (a) Erin Brockovich found that Cr(VI), used by PG&E, had contaminated the Hinckley, California, water supply. (b) The Cr(VI) ion is often present in water as the polyatomic ions chromate, CrO42−CrO42− (left), and dichromate, Cr2O72−Cr2O72− (right).

    Figure A shows a photo of Erin Brockovich. Figure B shows a 3-D ball-and-stick model of chromate. Chromate has a chromium atom at its center that forms bonds with four oxygen atoms each. Two of the oxygen atoms form single bonds with the chromium atom while the other two form double bonds each. The structure of dichromate consists of two chromate ions that are bonded and share one of their oxygen atoms to which each chromate atom has a single bond.

    Chromium compounds are widely used in industry, such as for chrome plating, in dye-making, as preservatives, and to prevent corrosion in cooling tower water, as occurred near Hinckley. In the environment, chromium exists primarily in either the Cr(III) or Cr(VI) forms. Cr(III), an ingredient of many vitamin and nutritional supplements, forms compounds that are not very soluble in water, and it has low toxicity. But Cr(VI) is much more toxic and forms compounds that are reasonably soluble in water. Exposure to small amounts of Cr(VI) can lead to damage of the respiratory, gastrointestinal, and immune systems, as well as the kidneys, liver, blood, and skin.

    Despite cleanup efforts, Cr(VI) groundwater contamination remains a problem in Hinckley and other locations across the globe. A 2010 study by the Environmental Working Group found that of 35 US cities tested, 31 had higher levels of Cr(VI) in their tap water than the public health goal of 0.02 parts per billion set by the California Environmental Protection Agency.

    Molecular (Covalent) Compounds

    The bonding characteristics of inorganic molecular compounds are different from ionic compounds, and they are named using a different system as well. The charges of cations and anions dictate their ratios in ionic compounds, so specifying the names of the ions provides sufficient information to determine chemical formulas. However, because covalent bonding allows for significant variation in the combination ratios of the atoms in a molecule, the names for molecular compounds must explicitly identify these ratios.

    Compounds Composed of Two Elements

    When two nonmetallic elements form a molecular compound, several combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO2. Since these are different substances with different properties, they cannot both have the same name (they cannot both be called carbon oxide). To deal with this situation, we use a naming method that is somewhat similar to that used for ionic compounds, but with added prefixes to specify the numbers of atoms of each element. The name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is first, followed by the name of the more nonmetallic element (the one farther to the right and/or top) with its ending changed to the suffix –ide. The numbers of atoms of each element are designated by the Greek prefixes shown in Table 14.5.

    When only one atom of the first element is present, the prefix mono- is usually deleted from that part. Thus, CO is named carbon monoxide, and CO2 is called carbon dioxide. When two vowels are adjacent, the a in the Greek prefix is usually dropped. Some other examples are shown in Table 14.6.

    Table 14.6

    Names of Some Molecular Compounds Composed of Two Elements

    CompoundName CompoundName
    SO2sulfur dioxide BCl3boron trichloride
    SO3sulfur trioxideSF6sulfur hexafluoride
    NO2nitrogen dioxidePF5phosphorus pentafluoride
    N2O4dinitrogen tetroxideP4O10tetraphosphorus decaoxide
    N2O5dinitrogen pentoxideIF7iodine heptafluoride

    There are a few common names that you will encounter as you continue your study of chemistry. For example, although NO is often called nitric oxide, its proper name is nitrogen monoxide. Similarly, N2O is known as nitrous oxide even though our rules would specify the name dinitrogen monoxide. (And H2O is usually called water, not dihydrogen monoxide.) You should commit to memory the common names of compounds as you encounter them.

    Example 14.2

    Naming Covalent Compounds

    Name the following covalent compounds:

    (a) SF6

    (b) N2O3

    (c) Cl2O7

    (d) P4O6


    Because these compounds consist solely of nonmetals, we use prefixes to designate the number of atoms of each element:

    (a) sulfur hexafluoride

    (b) dinitrogen trioxide

    (c) dichlorine heptoxide

    (d) tetraphosphorus hexoxide

    Check Your Learning

    Write the formulas for the following compounds:

    (a) phosphorus pentachloride

    (b) dinitrogen monoxide

    (c) iodine heptafluoride

    (d) carbon tetrachloride

    (a) PCl5; (b) N2O; (c) IF7; (d) CCl4

    This flowchart may help you organize all the information about nomenclature.

    Binary Acids

    Some compounds containing hydrogen are members of an important class of substances known as acids. The chemistry of these compounds is explored in more detail in later chapters of this text, but for now, it will suffice to note that many acids release hydrogen ions, H+, when dissolved in water. To denote this distinct chemical property, a mixture of water with an acid is given a name derived from the compound’s name. If the compound is a binary acid (comprised of hydrogen and one other nonmetallic element):

    1. The word “hydrogen” is changed to the prefix hydro-
    2. The other nonmetallic element name is modified by adding the suffix -ic
    3. The word “acid” is added as a second word

    For example, when the gas HCl (hydrogen chloride) is dissolved in water, the solution is called hydrochloric acid. Several other examples of this nomenclature are shown in Table 14.5.

    Table 14.5

    Names of Some Simple Acids

    Name of GasName of Acid
    HF(g), hydrogen fluorideHF(aq), hydrofluoric acid
    HCl(g), hydrogen chlorideHCl(aq), hydrochloric acid
    HBr(g), hydrogen bromideHBr(aq), hydrobromic acid
    HI(g), hydrogen iodideHI(aq), hydroiodic acid
    H2S(g), hydrogen sulfideH2S(aq), hydrosulfuric acid


    Many compounds containing three or more elements (such as organic compounds or coordination compounds) are subject to specialized nomenclature rules that you will learn later. However, we will briefly discuss the important compounds known as oxyacids, compounds that contain hydrogen, oxygen, and at least one other element, and are bonded in such a way as to impart acidic properties to the compound (you will learn the details of this in a later chapter). Typical oxyacids consist of hydrogen combined with a polyatomic, oxygen-containing ion. To name oxyacids:

    1. Omit “hydrogen”
    2. Start with the root name of the anion
    3. Replace –ate with –ic, or –ite with –ous
    4. Add “acid”

    For example, consider H2CO3 (which you might be tempted to call “hydrogen carbonate”). To name this correctly, “hydrogen” is omitted; the –ate of carbonate is replace with –ic; and acid is added—so its name is carbonic acid. Other examples are given in Table 14.6. There are some exceptions to the general naming method (e.g., H2SO4 is called sulfuric acid, not sulfic acid, and H2SO3 is sulfurous, not sulfous, acid).

    Table 14.6

    Names of Common Oxyacids

    FormulaAnion NameAcid Name
    HC2H3O2acetateacetic acid
    HNO3nitratenitric acid
    HNO2nitritenitrous acid
    HClO4perchlorateperchloric acid
    H2CO3carbonatecarbonic acid
    H2SO4sulfatesulfuric acid
    H2SO3sulfitesulfurous acid
    H3PO4phosphatephosphoric acid

    Link to Supplemental Exercises

    Supplemental exercises are available if you would like more practice with these concepts.


    Previous Citation(s)
    Flowers, P., Neth, E. J., Robinson, W. R., Theopold, K., & Langley, R. (2019). Chemistry in Context. In Chemistry: Atoms First 2e. OpenStax. https://openstax.org/books/chemistry-atoms-first-2e/pages/4-3-chemical-nomenclature

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