3.13: Ionic Bonding: Writing Chemical Names of Ionic Compounds Containing Polyatomic Ions

Learning Objectives

• Write the chemical names of ionic compounds containing polyatomic ions.

In the previous section, the process for writing the chemical formula of an ionic compound containing a polyatomic ion was presented and applied.  The chemical name of a compound is derived based on the information included in its chemical formula, and no two chemical formulas should share a common chemical name.  As several polyatomic ions exist as part of a related series, specialized "-ate" and "-ite" suffixes are employed to indicate the relative number of oxygens that are contained within these ions.  Since the names of polyatomic ions cannot be modified in any way, these suffixes must also be incorporated into the chemical name of an ionic compound that contains a polyatomic ion, as will be explained in greater detail in the following paragraphs.

Naming Ionic Compounds Containing Polyatomic Ions

The chemical name of an ionic compound is based solely on the identities of the ions that it contains.  Specifically, the names of the ions are modified by removing the word "ion" from each, and the remaining terms are written in the order in which they appear in the corresponding ionic chemical formula.  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 redundant.  Therefore, ionic compounds do not include any numerical prefixes.  

For example, consider Li₂SO₄, which is the chemical formula for the ionic compound that is formed when the sulfate ion and lithium bond with one another.

These elements bond with one another as ions, not as neutral atoms.  Therefore, more accurately, Li₂SO₄ is the chemical formula for the ionic compound that is formed when the sulfate ion (SO₄^–2, a polyatomic anion) and the lithium ion (Li⁺¹, the cation formed when lithium ionizes) bond with one another.  Recall that the suffix of a monatomic anion is "-ide," as a verbal indicator of its negative charge.  However, the name of a polyatomic ion is defined by and, therefore, is integral to, the identity of the ion and cannot be altered in any way.  Furthermore, the sulfate ion is one of the polyatomic ions that exists as part of a related series, as denoted by its "-ate" suffix.  This suffix indicates the greater number of oxygens that are contained within this polyatomic ion, relative to the other ion in the related series, the sulfite ion (SO₃^–2).  Therefore, this ending cannot be modified to the "-ide" suffix that is typically indicative of negatively-charged particles.

When naming an ionic compound, the word "ion" is removed from both the cation and the anion terms, as no charges are explicitly-written in an ionic chemical formula.  Each constituent particle, such as SO₄^–2 and Li⁺¹, is charged and, consequently, has a name that includes the word "ion."  However, an ionic compound, such as Li₂SO₄, is a net-neutral species, due to the charge-balance achieved between these particles.  Therefore, the term "ion" should not be incorporated into the chemical name of an ionic compound.  In this example, "sulfate ion" is shortened to "sulfate," and "lithium ion" becomes "lithium."

Finally, since the cation is symbolized before the anion in an ionic chemical formula, the cation term appears first in the chemical name of an ionic compound.  Therefore, in this example, the term "lithium" is written before "sulfate."  As the subscripts in an ionic chemical formula are not referenced in an ionic chemical name, the result of combining these terms, "lithium sulfate," is the chemically-correct name for Li₂SO₄.

Example \(\PageIndex{1}\)

Write the chemical name of Be(CN)₂, the ionic compound that is formed when beryllium and the cyanide ion bond with one another.

Solution

More accurately, Be(CN)₂ is the chemical formula for the ionic compound that is formed when the beryllium ion (Be⁺², the cation formed when beryllium ionizes) and the cyanide ion (CN^–1, a polyatomic anion) bond with one another.  Recall that the suffix of a monatomic anion is "-ide," as a verbal indicator of its negative charge, but the name of a polyatomic ion is defined by and, therefore, is integral to, the identity of the ion and cannot be altered in any way.

When naming an ionic compound, the word "ion" is removed from both the cation and the anion terms, as no charges are explicitly-written in an ionic chemical formula.  As a result, "cyanide ion" is shortened to "cyanide," and "beryllium ion" becomes "beryllium."  Finally, since the cation is symbolized before the anion in an ionic chemical formula, the cation term appears first in the chemical name of an ionic compound.  Therefore, in this example, the word "beryllium" is written before "cyanide."  As the subscripts in an ionic chemical formula are not referenced in an ionic chemical name, the result of combining these terms, "beryllium cyanide," is the chemically-correct name for Be(CN)₂.

Exercise \(\PageIndex{1}\)

Write the chemical name that corresponds to each of the following chemical formulas. 

1. (NH₄)₃PO₃, the ionic compound that is formed when the ammonium ion and the phosphite ion bond with one another.
2. HgCO₃, an ionic compound that is formed when the carbonate ion and mercury bond with one another.

Answer a

Recall that the suffix of a monatomic anion is "-ide," as a verbal indicator of its negative charge, but the name of a polyatomic ion is defined by and, therefore, is integral to, the identity of the ion and cannot be altered in any way.  The phosphite ion (PO₃^–3) is one of the polyatomic ions that exists as part of a related series, as denoted by its "-ite" suffix.  This suffix indicates the lesser number of oxygens that are contained within this polyatomic ion, relative to the other ion in the related series, the phosphate ion (PO₄^–3).  Therefore, this ending cannot be modified to the "-ide" suffix that is typically indicative of negatively-charged particles.

When naming an ionic compound, the word "ion" is removed from both the cation and the anion terms, as no charges are explicitly-written in an ionic chemical formula.  As a result, "phosphite ion" is shortened to "phosphite," and "ammonium ion" becomes "ammonium."  Finally, since the cation is symbolized before the anion in an ionic chemical formula, the cation term appears first in the chemical name of an ionic compound.  Therefore, in this example, the term "ammonium" is written before "phosphite."  As the subscripts in an ionic chemical formula are not referenced in an ionic chemical name, the result of combining these terms, "ammonium phosphite," is the chemically-correct name for (NH₄)₃PO₃.

Answer b

More accurately, HgCO₃ is the chemical formula for an ionic compound that is formed when the carbonate ion (CO₃^–2, a polyatomic anion) and a mercury ion bond with one another.  Recall that the suffix of a monatomic anion is "-ide," as a verbal indicator of its negative charge, but the name of a polyatomic ion is defined by and, therefore, is integral to, the identity of the ion and cannot be altered in any way.  Therefore, the "-ate" ending within "carbonate ion" cannot be modified to the "-ide" suffix that is typically indicative of negatively-charged particles.

Additionally, mercury (Hg) is able to achieve a stable electron configuration through multiple ionization pathways and could ionize to form either Hg₂⁺², which is the chemically-correct formula for the mercury (I) ion, or Hg⁺².  Therefore, the specific charge of the transition metal cation must be definitively established before an unambiguous ionic chemical name can be written.

An adaptation of the "Ratio Method," which was utilized to determine the subscripts in ionic chemical formulas, can be reliably modified and employed for this task.  Recall that this process establishes a cation-to-anion ratio by equating the total charges of the cations in an ionic compound to the sum of the charges of the anions in that compound, in order to ensure that the final compound is a net-neutral species.  As the "Reverse Ratio Method" requires the use of a relative cation-to-anion ratio, using subscripts that have been reduced to a lowest-common ratio of whole numbers will consistently indicate the correct charge of a transition metal cation.  In this procedure, a mini-equation, in which the subscript on the cation is multiplied by a variable, such as x, and the subscript on the anion is multiplied by the absolute value of its charge, is solved.  Note that the charge of the anion is always a known quantity, as all anions are derived from main group elements or polyatomic ions, which have defined, predictable charges.  Additionally, the absolute value of the anion charge is used, as anions are negative, and this procedure is being employed to find the charge of a cation, which is positive.  Finally, an absolute value is represented by writing a quantity inside of two vertical bars.  When solving equations that involve absolute values, a positive value is applied in subsequent mathematical operations.

Determining the subscript on the carbonate ion (CO₃^–2, a polyatomic anion with a defined charge of –2) is the most challenging aspect of this example.  As no parentheses are explicitly-written around the "CO₃" portion of the ionic chemical formula, only a single polyatomic ion is contained in this compound.  The explicitly-written subscript, a 3, refers to the total number oxygens contained within the carbonate ion unit, not the overall polyatomic ion, itself.  Therefore, in the current example, the subscripts on both the cation, mercury (Hg) and on the carbonate ion are unwritten "1"s.  Integrating this information into the mini-equation described in the previous paragraph yields 

1(x) = 1(|–2|)
1(x) = 1(2)
x = 2

This result indicates that the mercury cation in this example has a charge of +2 and, therefore, is symbolized as Hg⁺².

After establishing the correct charge of the transition metal cation, the appropriate Roman numeral can be incorporated into its ion name.  Recall that Roman numerals should be written in parentheses after the element name, but before the word "ion."  The +2 charge of this mercury cation is represented by a Roman numeral (II) in an ion name.  Therefore, the unambiguous name of "Hg⁺²" is the mercury (II) ion.

When naming an ionic compound, the word "ion" is removed from both the cation and the anion terms, as no charges are explicitly-written in an ionic chemical formula.  As a result, "carbonate ion" is shortened to "carbonate," and "mercury (II) ion" becomes "mercury (II)."  Finally, the cation term appears first in the chemical name of an ionic compound, so the phrase "mercury (II)" is written before "carbonate."  As the subscripts in an ionic chemical formula are not referenced in an ionic chemical name, the result of combining these terms, "mercury (II) carbonate," is the chemically-correct name for HgCO₃.