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8.11: Arrhenius Acids and Bases: Writing Chemical Formulas and Names of Arrhenius Bases

  • Page ID
    226549
  • Learning Objectives
    • Write chemical formulas of Arrhenius bases.
    • Name Arrhenius bases.

    As stated previously, an Arrhenius base must, by definition, contain a hydroxide ion, OH–1and a component that ionizes to produce a stable, positively-charged particle.  Furthermore, because hydroxide ions are present in all Arrhenius bases, the chemical formula and name of a specific Arrhenius base is dependent on the type of cation that it contains.  However, since very few stable polyatomic cations exist, these positively-charged units are not commonly found in Arrhenius bases.  Therefore, because all Arrhenius bases contain monatomic, or single-atom, cations, only one "type" of Arrhenius base exists.  Consequently, these compounds do not need additional qualifying descriptors to indicate the chemical composition of their constituent cations and, therefore, are referred to as "Arrhenius bases."  The following paragraphs will present and apply the pattern for determining the chemical formulas and names of Arrhenius bases that contain monatomic, or single-atom, cations.

    Writing Chemical Formulas of Arrhenius Bases

    All of the metals that are present on the periodic table lose valence electrons to achieve octet configurations and, consequently, ionize to form monatomic, or single-atom, cations, which are collectively symbolized as "M+Y."  In this notation, the elemental symbol of the metal is represented as "M," and "Y" corresponds to the numerical charge of the cation that results upon the ionization of that metal.

    Based on the bonding combinations that were presented in Chapter 3, polyatomic anions, such as the hydroxide ion, OH–1, will interact with metals, which ionize to produce cations, to form ionic compounds.  Therefore, the chemical formula of an Arrhenius base that contains hydroxide ions and a monatomic cation can be derived by applying the Chapter 3 rules for determining ionic chemical formulas.

    Recall that the symbol for the cationic component of an ionic compound is written first in an ionic chemical formula, and that, after removing the "+" and "–" signs from each of the constituent ion symbols, the subscripts in an ionic base formula are derived using either the "Ratio Method" or the "Criss-Cross Method."  Because the hydroxide ion, OH–1, is a polyatomic ion and, therefore, bonds as an indivisible unit, its chemical formula must be enclosed inside of two parentheses when incorporated into an ionic chemical formula, and the subscript that specifies how many hydroxide ions are present within the corresponding compound must be written after the closing parenthesis.  The subscripts in the base formula must be reduced to the lowest-common ratio of whole numbers, if possible, and any explicitly-written "1"s must be removed.  Finally, if no subscript is present after the closing parentheses in the resultant chemical formula, the parentheses surrounding the polyatomic ion should be removed.

    By applying these rules to the cationic, M+Y, and anionic, OH–1, components of an Arrhenius base, a formula pattern of "M(OH)Y" results.  Since the hydroxide ion, OH–1, bears a –1 charge, the subscript that is written to indicate the number of metal cations that are present in the corresponding compound has a value of "1," which should not be explicitly-written in the chemical formula that is being developed.  Therefore, an Arrhenius base is understood to contain one metal cation, M+Y.  Additionally, recall that the charge of a metal cation corresponds to the number of valence electrons that are lost during the ionization of a metallic atom, which, in turn, varies based on the column, or Group, in which an element is located on the periodic table.  Therefore, the metals in Group 1/1A, 2/2A, and 13/3A on the periodic table ionize to form cations with +1, +2, and +3 charges, respectively.  Furthermore, because transition metals can lose inner shell electrons, in addition to their valence electrons, most transition metals are able to achieve stable electron configurations through multiple ionization pathways and, therefore, ionize to form several unique cations.  Consequently, in order to achieve charge-balance in an Arrhenius base, which, as stated above, is understood to contain one metal cation, M+Y, the number of hydroxide ions, OH–1, that are present in the final compound must be equal to the charge, Y, of the metal cation.  Therefore, to obtain the chemical formula of a particular Arrhenius base, the "M" and "Y" portions of the pattern that is shown above can be replaced with the elemental symbol of a specific metal and the numerical charge of the cation that results upon the ionization of that metal, respectively.

    For example, write the chemical formula of the Arrhenius base that contains a magnesium ion.

    As stated above, an Arrhenius base, which can be generically-symbolized as "M(OH)Y," contains a hydroxide ion, OH–1, and a monatomic metallic cation, by definition.  In order to obtain the chemical formula of a particular Arrhenius base, the "M" and "Y" portions of the pattern that is shown above can be replaced with the elemental symbol of a specific metal and the numerical charge of the cation that results upon the ionization of that metal, respectively.  Because the indicated cation, a magnesium ion, Mg+2, bears a +2 charge, the corresponding Arrhenius base must contain 2 hydroxide ions, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius base that contains a magnesium ion, Mg+2, is Mg(OH)2.

    Exercise \(\PageIndex{1}\)

    Write the chemical formula of the Arrhenius base that contains

    1. a lead (IV) ion.
    2. a lithium ion.
    3. an aluminum ion.
    4. a copper (I) ion
    Answer a
    In order to obtain the chemical formula of a particular Arrhenius base, the "M" and "Y" portions of the generic "M(OH)Y" formula pattern can be replaced with the elemental symbol of a specific metal and the numerical charge of the cation that results upon the ionization of that metal, respectively.  Because the indicated cation, a lead (IV) ion, Pb+4, is a transition metal, the Roman numeral in the given name indicates that this ion bears a +4 charge.  As a result, the corresponding Arrhenius base must contain 4 hydroxide ions, in order to achieve charge-balance in the final compound, and, therefore, the chemical formula of the Arrhenius base that contains a lead (IV) ion, Pb+4, is Pb(OH)4.
    Answer b
    In order to obtain the chemical formula of a particular Arrhenius base, the "M" and "Y" portions of the generic "M(OH)Y" formula pattern can be replaced with the elemental symbol of a specific metal and the numerical charge of the cation that results upon the ionization of that metal, respectively.  Because the indicated cation, a lithium ion, Li+1, bears a +1 charge, the corresponding Arrhenius base must contain 1 hydroxide ion, in order to achieve charge-balance in the final compound.  However, since values of "1" are usually implicitly-understood in chemistry, a subscript that corresponds to this ion count should not be written in the formula that is being developed, and, consequently, the parentheses surrounding the chemical formula of the hydroxide ion should also be eliminated.  Therefore, the chemical formula of the Arrhenius base that contains a lithium ion, Li+1, is LiOH.
    Answer c
    In order to obtain the chemical formula of a particular Arrhenius base, the "M" and "Y" portions of the generic "M(OH)Y" formula pattern can be replaced with the elemental symbol of a specific metal and the numerical charge of the cation that results upon the ionization of that metal, respectively.  Because the indicated cation, an aluminum ion, Al+3, bears a +3 charge, the corresponding Arrhenius base must contain 3 hydroxide ions, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius base that contains an aluminum ion, Al+3, is Al(OH)3.
    Answer d
    In order to obtain the chemical formula of a particular Arrhenius base, the "M" and "Y" portions of the generic "M(OH)Y" formula pattern can be replaced with the elemental symbol of a specific metal and the numerical charge of the cation that results upon the ionization of that metal, respectively.  Because the indicated cation, a copper (I) ion, Cu+1, is a transition metal, the Roman numeral in the given name indicates that this ion bears a +1 charge.  As a result, the corresponding Arrhenius base must contain 1 hydroxide ion, in order to achieve charge-balance in the final compound.  However, since values of "1" are usually implicitly-understood in chemistry, a subscript that corresponds to this ion count should not be written in the formula that is being developed, and, consequently, the parentheses surrounding the chemical formula of the hydroxide ion should also be eliminated.  Therefore, the chemical formula of the Arrhenius base that contains a copper (I) ion, Cu+1, is CuOH.

    Naming Arrhenius Bases

    The name of an ionic compound is based solely on the identities of the ions that it contains.  Since the subscripts in an ionic chemical formula are the result of achieving charge-balance between the compound's constituent ions, referencing subscripts in an ionic chemical name is considered redundant.  Therefore, the names of ionic compounds do not include any numerical prefixes.

    As stated above, because all Arrhenius bases contain one or more hydroxide ions, OH–1, the name of a specific Arrhenius base is dependent on the identity of the metal that it contains.  Since the cation is symbolized before the anion in an ionic chemical formula, the name of the cation must appear first in the name of the corresponding compound, and the word "ion" should be removed from each ion name, as no charges are explicitly-written in an ionic chemical formula.  Finally, because most transition metals are able to achieve stable electron configurations through multiple ionization pathways and, therefore, ionize to form several unique cations, the charge of a transition metal cation that is present in an Arrhenius base must be specified by adding a corresponding Roman numeral, which should be written in parentheses, to the name of the metal cation.  Finally, because polyatomic anions, such as the hydroxide ion, OH–1, always interact with metals to form ionic compounds, the suffixes of the remaining cation and anion terms should botbe incorporated, as-written, into the name of an Arrhenius base.

    For example, write the name of the Arrhenius base that is symbolized as Mg(OH)2.

    When naming an Arrhenius base, the name of the cation must be written before the name of the anion, the word "ion" should be removed from each of these names, and the suffixes of botnames should be incorporated, as-written, into the name that is being developed.  Therefore, because the molecule that is symbolized above contains a magnesium ionMg+2, and two hydroxide ions, OH–1, the name of the given molecule is magnesium hydroxide.

    Exercise \(\PageIndex{2}\)

    Write the name of the Arrhenius base that is symbolized as

    1. Pb(OH)4.
    2. LiOH.
    3. Al(OH)3.
    4. CuOH.
    Answer a
    When naming an Arrhenius base, the name of the cation must be written before the name of the anion, the word "ion" should be removed from each of these names, and the suffixes of both of the remaining words should be incorporated, as-written, into the name that is being developed.

    Lead, the metal that is present in the given chemical formula, is a transition metal and, therefore, is able to achieve a stable electron configuration through multiple ionization pathways.  Because lead could ionize to form either Pb+2 or Pb+4, the specific charge of the lead cation that is contained in Pb(OH)4 must be definitively established before an unambiguous ionic chemical name can be written.  The "Reverse Ratio Method" that was described in Section 3.10 could be utilized to determine this charge information.  Alternatively, recall that the "Y" subscript that is present in the Arrhenius base formula pattern, "M(OH)Y," corresponds to the numerical charge of the cation that is contained in a particular base.  Therefore, because the numerical value of "Y" in the given chemical formula, Pb(OH)4, is a "4," the lead ion that is present in the corresponding Arrhenius base bears a +4 charge and, therefore, can be symbolized as Pb+4.  This charge information is specified in the name of the Arrhenius base by adding a Roman numeral, (IV), to the name of the transition metal cation.

    Therefore, because the molecule that is symbolized above contains a lead (IV) ionPb+4, and four hydroxide ions, OH–1, the name of the given molecule is lead (IV) hydroxide.
    Answer b
    When naming an Arrhenius base, the name of the cation must be written before the name of the anion, the word "ion" should be removed from each of these names, and the suffixes of both of the remaining words should be incorporated, as-written, into the name that is being developed.  Therefore, because the molecule that is symbolized above contains a lithium ionLi+1, and a hydroxide ion, OH–1, the name of the given molecule is lithium hydroxide.
    Answer c
    When naming an Arrhenius base, the name of the cation must be written before the name of the anion, the word "ion" should be removed from each of these names, and the suffixes of both of the remaining words should be incorporated, as-written, into the name that is being developed.  Therefore, because the molecule that is symbolized above contains an aluminum ionAl+3, and three hydroxide ions, OH–1, the name of the given molecule is aluminum hydroxide.
    Answer d
    When naming an Arrhenius base, the name of the cation must be written before the name of the anion, the word "ion" should be removed from each of these names, and the suffixes of botof the remaining words should be incorporated, as-written, into the name that is being developed.

    Copper, the metal that is present in the given chemical formula, is a transition metal and, therefore, is able to achieve a stable electron configuration through multiple ionization pathways.  Because copper could ionize to form either Cu+1 or Cu+2, the specific charge of the copper cation that is contained in CuOH must be definitively established before an unambiguous ionic chemical name can be written.  The "Reverse Ratio Method" that was described in Section 3.10 could be utilized to determine this charge information.  Alternatively, recall that the "Y" subscript that is present in the Arrhenius base formula pattern, "M(OH)Y," corresponds to the numerical charge of the cation that is contained in a particular base.  Therefore, because the numerical value of "Y" in the given chemical formula, CuOH, is an unwritten "1," the copper ion that is present in the corresponding Arrhenius base bears a +1 charge and, therefore, can be symbolized as Cu+1.  This charge information is specified in the name of the Arrhenius base by adding a Roman numeral, (I), to the name of the transition metal cation.

    Therefore, because the molecule that is symbolized above contains a copper (I) ionCu+1, and a hydroxide ion, OH–1, the name of the given molecule is copper (Ihydroxide.