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3.5 Covalently-Bonded Species: Formulas and Names

  • Page ID
    159929
  • Skills to Develop

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

    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 \(\PageIndex{3}\).

    Table \(\PageIndex{3}\): Nomenclature Prefixes
    Number Prefix   Number Prefix
    1 (sometimes omitted) mono-   6 hexa-
    2 di- 7 hepta-
    3 tri- 8 octa-
    4 tetra- 9 nona-
    5 penta- 10 deca-

    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 \(\PageIndex{4}\).

    Table \(\PageIndex{4}\): Names of Some Molecular Compounds Composed of Two Elements
    Compound Name   Compound Name
    SO2 sulfur dioxide   BCl3 boron trichloride
    SO3 sulfur trioxide SF6 sulfur hexafluoride
    NO2 nitrogen dioxide PF5 phosphorus pentafluoride
    N2O4 dinitrogen tetroxide P4O10 tetraphosphorus decaoxide
    N2O5 dinitrogen pentoxide IF7 iodine 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. You should commit to memory the common names of compounds as you encounter them.

    Example \(\PageIndex{2}\): Naming Covalent Compounds

    Name the following covalent compounds:

    1. SF6
    2. N2O3
    3. Cl2O7
    4. P4O6

    Solution

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

    1. sulfur hexafluoride
    2. dinitrogen trioxide
    3. dichlorine heptoxide
    4. tetraphosphorus hexoxide

    Exercise \(\PageIndex{2}\)

    Write the formulas for the following compounds:

    1. phosphorus pentachloride
    2. dinitrogen monoxide
    3. iodine heptafluoride
    4. carbon tetrachloride

    Answer:

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

    Binary Compounds of Hydrogen

    Covalently-bonded compounds that consist of only H atoms and one atom of one other element are named as if they are ionically-bonded. Specifically, the number of atoms is not indicated in the name. Thus, HI is named hydrogen iodide, and H2S is named hydrogen sulfide. Be aware that the systematic name is not dihydrogen monoxide.

    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 \(\PageIndex{5}\).

    Table \(\PageIndex{5}\): Names of Some Simple Acids
    Name of Gas Name of Acid
    HF(g), hydrogen fluoride HF(aq), hydrofluoric acid
    HCl(g), hydrogen chloride HCl(aq), hydrochloric acid
    HBr(g), hydrogen bromide HBr(aq), hydrobromic acid
    HI(g), hydrogen iodide HI(aq), hydroiodic acid
    H2S(g), hydrogen sulfide H2S(aq), hydrosulfuric acid

    Oxyacids

    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 \(\PageIndex{6}\). 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 \(\PageIndex{6}\): Names of Common Oxyacids
    Formula Anion Name Acid Name
    HC2H3O2 acetate acetic acid
    HNO3 nitrate nitric acid
    HNO2 nitrite nitrous acid
    HClO4 perchlorate perchloric acid
    H2CO3 carbonate carbonic acid
    H2SO4 sulfate sulfuric acid
    H2SO3 sulfite sulfurous acid
    H3PO4 phosphate phosphoric acid

    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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