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3.4: Ionic Nomenclature

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  • Page ID
    68029
  • Skills to Develop

    • To use the rules for naming ionic compounds

    After learning a few more details about the names of individual ions, you will be a step away from knowing how to name ionic compounds. This section begins the formal study of nomenclature, the systematic naming of chemical compounds.

    Naming Ions

    The name of a monatomic cation is simply the name of the element followed by the word ion. Thus, Na+ is the sodium ion, Al3+ is the aluminum ion, Ca2+ is the calcium ion, and so forth.

    We have seen that some elements lose different numbers of electrons, producing ions of different charges (Figure 3.3). Iron, for example, can form two cations, each of which, when combined with the same anion, makes a different compound with unique physical and chemical properties. Thus, we need a different name for each iron ion to distinguish Fe2+ from Fe3+. The same issue arises for other ions with more than one possible charge.

    There are two ways to make this distinction. In the simpler, more modern approach, called the Stock system, an ion’s positive charge is indicated by a roman numeral in parentheses after the element name, followed by the word ion. Thus, Fe2+ is called the iron(II) ion, while Fe3+ is called the iron(III) ion. This system is used only for elements that form more than one common positive ion. We do not call the Na+ ion the sodium(I) ion because (I) is unnecessary. Sodium forms only a 1+ ion, so there is no ambiguity about the name sodium ion.

    The second system, called the common system, is not conventional but is still prevalent and used in the health sciences. This system recognizes that many metals have two common cations. The common system uses two suffixes (-ic and -ous) that are appended to the stem of the element name. The -ic suffix represents the greater of the two cation charges, and the -ous suffix represents the lower one. In many cases, the stem of the element name comes from the Latin name of the element. Table \(\PageIndex{1}\) lists the elements that use the common system, along with their respective cation names.

    Table \(\PageIndex{1}\): The Common System of Cation Names
    Element Stem Charge Name
    iron fe rr- 2+ ferrous ion
    3+ ferric ion
    copper cupr- 1+ cuprous ion
    2+ cupric ion

    The name of a monatomic anion consists of the stem of the element name, the suffix -ide, and then the word ion. Thus, as we have already seen, Cl is “chlor-” + “-ide ion,” or the chloride ion. Similarly, O2− is the oxide ion, Se2− is the selenide ion, and so forth. Table \(\PageIndex{2}\) lists the names of some common monatomic ions.

    Table \(\PageIndex{2}\): Some Monatomic Anions
    Ion Name
    F fluoride ion
    Cl chloride ion
    Br bromide ion
    I iodide ion
    O2− oxide ion
    S2− sulfide ion
    N3− nitride ion

    The polyatomic ions have their own characteristic names, as discussed earlier.

    Example \(\PageIndex{1}\)

    Name each ion.

    1. Ca2+
    2. S2−
    3. SO32−
    4. NH4+
    5. Cu+

    SOLUTION

    1. the calcium ion
    2. the sulfide ion (from Table \(\PageIndex{2}\))
    3. the sulfite ion 
    4. the ammonium ion 
    5. the copper(I) ion or the cuprous ion (copper can form cations with either a 1+ or 2+ charge, so we have to specify which charge this ion has)

    Exercise \(\PageIndex{1}\)

    Name each ion.

    1. Fe2+
    2. Fe3+
    3. SO42−
    4. Ba2+
    5. HCO3

    Example \(\PageIndex{2}\)

    Write the formula for each ion.

    1. the bromide ion
    2. the phosphate ion
    3. the cupric ion
    4. the magnesium ion

    SOLUTION

    1. Br
    2. PO43−
    3. Cu2+
    4. Mg2+

    Exercise \(\PageIndex{2}\)

    Write the formula for each ion.

    1. the fluoride ion
    2. the carbonate ion
    3. the ferricion
    4. the potassium ion

    Naming Compounds

    Now that we know how to name ions, we are ready to name ionic compounds. We do so by placing the name of the cation first, followed by the name of the anion, and dropping the word ion from both parts. For example, what is the name of the compound whose formula is Ba(NO3)2?

    Ba(NO3)2.jpg

    The compound’s name does not indicate that there are two nitrate ions for every barium ion. You must determine the relative numbers of ions by balancing the positive and negative charges.

    If you are given a formula for an ionic compound whose cation can have more than one possible charge, you must first determine the charge on the cation before identifying its correct name. For example, consider FeCl2 and FeCl3. In the first compound, the iron ion has a 2+ charge because there are two Cl ions in the formula (1− charge on each chloride ion). In the second compound, the iron ion has a 3+ charge, as indicated by the three Cl ions in the formula. These are two different compounds that need two different names. By the Stock system, the names are iron(II) chloride and iron(III) chloride. If we were to use the stems and suffixes of the common system, the names would be ferrous chloride and ferric chloride, respectively.

    Example \(\PageIndex{3}\)

    Name each ionic compound, using both Stock and common systems if necessary.

    1. Ca3(PO4)2
    2. (NH4)2Cr2O7
    3. KCl
    4. CuCl
    5. FeF2

    SOLUTION

    1. calcium phosphate
    2. ammonium dichromate (the prefix di- is part of the name of the anion)
    3. potassium chloride
    4. copper(I) chloride or cuprous chloride
    5. iron (II) fluoride or ferrous fluoride

    Exercise \(\PageIndex{3}\)

    Name each ionic compound, using both Stock and common systems if necessary.

    1. ZnBr2
    2. Fe(NO3)3
    3. Al2O3

    Figure \(\PageIndex{1}\) is a synopsis of how to name simple ionic compounds.

    3.7.jpg

    Figure \(\PageIndex{1}\): A Guide to Naming Simple Ionic Compounds. Follow these steps to name a simple ionic compound.

    KEY TAKEAWAY

    • Each ionic compound has its own unique name that comes from the names of the ions.

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