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6.5: Naming Ionic Compounds

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    Learning Objectives
    • Given the formula of an ionic compound (which may contain a fixed-charge or variable-charge metal and may contain a polyatomic ion) write the name.

    Naming Binary Ionic Compounds with a Metal that Forms Only One Type of Cation

    A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. The metal cation is named first, followed by the nonmetal anion as illustrated in Figure \(\PageIndex{1}\) for the compound BaCl2. The word ion is dropped from both parts.

    Naming formula: Name of metal cation + base name of nonmetal anion + suffix -ide. BaCl2 is named as barium chloride.
    Figure \(\PageIndex{1}\): Naming \(BaCl_2\)

    Subscripts in the formula do not affect the name.

    Example \(\PageIndex{3}\): Naming Ionic Compounds

    Name each ionic compound.

    1. CaCl2
    2. AlF3
    3. KCl

    Solution

    1. Using the names of the ions, this ionic compound is named calcium chloride.
    2. The name of this ionic compound is aluminum fluoride.
    3. The name of this ionic compound is potassium chloride
    Exercise \(\PageIndex{3}\)

    Name each ionic compound.

    1. AgI
    2. MgO
    3. Ca3P2
    Answer a:
    silver iodide
    Answer b:
    magnesium oxide
    Answer c:
    calcium phosphide

    Naming Binary Ionic Compounds with a Metal That Forms More Than One Type of Cation

    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. The names are iron(II) chloride and iron(III) chloride (Figure \(\PageIndex{2}\)).

    Table \(\PageIndex{3}\): Naming the \(FeCl_2\) and \(FeCl_3\) Compounds in the Modern/Stock System.
    Name of cation (metal) + (Roman Numeral in parenthesis) + Base name of anion (nonmetal) and -ide
    FeCl2 is named as iron (II) chloride. FeCl3 is named as iron (III) chloride.

    Figure \(\PageIndex{2}\): Naming Ionic Compounds with Variable Charge Metals

     
    Example \(\PageIndex{4}\):

    Name each ionic compound.

    1. Co2O3
    2. FeCl2

    Solution

      Explanation Answer
    a

    We know that cobalt can have more than one possible charge; we just need to determine what it is.

    • Oxide always has a 2− charge, so with three oxide ions, we have a total negative charge of 6−.
    • This means that the two cobalt ions have to contribute 6+, which for two cobalt ions means that each one is 3+.
    • Therefore, the proper name for this ionic compound is cobalt(III) oxide.
    		 cobalt(III) oxide
    b

    Iron can also have more than one possible charge.

    • Chloride always has a 1− charge, so with two chloride ions, we have a total negative charge of 2−.
    • This means that the one iron ion must have a 2+ charge.
    • Therefore, the proper name for this ionic compound is iron(II) chloride.
    		 iron(II) chloride
    Exercise \(\PageIndex{4}\)

    Name each ionic compound.

    1. AuCl3
    2. PbO2
    3. CuO
    Answer a:
    gold(III) chloride
    Answer b:
    lead(IV) oxide
    Answer c:
    copper(II) oxide

    Naming Ionic Compounds with Polyatomic Ions

    The process of naming ionic compounds with polyatomic ions is the same as naming binary ionic compounds. The cation is named first, followed by the anion. One example is the ammonium sulfate compound in Figure \(\PageIndex{6}\).

    (NH4)2SO4 is named as ammonium sulfate.
    Figure \(\PageIndex{3}\): Naming Ionic Compounds with Polyatomic Ions
    Example \(\PageIndex{5}\): Naming Ionic Compounds

    Write the proper name for each ionic compound.

    1. (NH4)2S
    2. AlPO4,
    3. Fe3(PO4)2

    Solution

    Explanation Answer

    a. The ammonium ion has a 1+ charge and the sulfide ion has a 2− charge.

    Two ammonium ions need to balance the charge on a single sulfide ion.

    The compound’s name is ammonium sulfide.

    		 ammonium sulfide

    b. The ions have the same magnitude of charge, one of each (ion) is needed to balance the charges.

    The name of the compound is aluminum phosphate.

    		 aluminum phosphate

    c. Neither charge is an exact multiple of the other, so we have to go to the least common multiple of 6.

    To get 6+, three iron(II) ions are needed, and to get 6−, two phosphate ions are needed .

    The compound’s name is iron(II) phosphate.

    		 iron(II) phosphate
    Exercise \(\PageIndex{5A}\)

    Write the proper name for each ionic compound.

    1. (NH4)3PO4
    2. Co(NO2)3
    Answer a:
    ammonium phosphate
    Answer b:
    cobalt(III) nitrite

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

    3.7.jpg
    Figure \(\PageIndex{3}\): A Guide to Naming Simple Ionic Compounds.
    Exercise \(\PageIndex{5B}\)

    Name each ionic compound.

    1. ZnBr2
    2. Al2O3
    3. (NH4)3PO4
    4. AuF3
    5. AgF
    Answer a:
    zinc bromide
    Answer b:
    aluminum oxide
    Answer c:
    ammonium phosphate
    Answer d:
    gold(III) fluoride
    Answer e:
    silver fluoride

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