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.