In the previous section, the process for writing the chemical formula of an ionic compound containing a transition metal 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 a transition metal is able to achieve a stable electron configuration through multiple ionization pathways, its corresponding ion name must be modified using a Roman numeral to specify the charge of the particular cation that is formed. This information must also be incorporated into the chemical name of an ionic compound that contains a transition metal, as will be explained in greater detail in the following paragraphs.
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 Sn3P4, which is the chemical formula for an ionic compound that is formed when phosphorus and tin bond with one another.
These elements bond with one another as ions, not as neutral atoms. Therefore, more accurately, Sn3P4 is the chemical formula for the ionic compound that is formed when the phosphide ion, P–3, the anion formed upon the ionization of phosphorus, and a tin ion bond with one another. Recall that the suffix of an anion is "-ide," as a verbal indicator of its negative charge. However, tin, Sn, is able to achieve a stable electron configuration through multiple ionization pathways and could ionize to form either Sn+2 or Sn+4. 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 "Criss-Cross Method," which was utilized to determine the subscripts in ionic chemical formulas, can be used for this task. However, as the subscripts in ionic chemical formulas are a lowest-common ratio of whole numbers, the charges found by employing a "reverse version" of this "shortcut" procedure can be incorrect. Therefore, the "Reverse Criss-Cross Method" will not be discussed further in this section.
In contrast, the alternative process, the "Ratio Method," can be reliably modified and employed to determine the charge of a transition metal cation. 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 still 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, 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.
In the current example, the subscript on the cation, tin (Sn), is "3," and the subscript on the phosphide anion(P–3), which has a known charge of –3, is "4." Therefore,
3(x) = 4(|–3|) 3(x) = 4(3) x = 4
This result indicates that the tin cation in this example has a charge of +4 and, therefore, is symbolized as Sn+4.
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." Based on the results of the "Reverse Ratio Method," this tin cation has a charge of +4, which is represented by a Roman numeral (IV) in an ion name. Therefore, the unambiguous name of "Sn+4" is the tin (IV) 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. Each constituent particle, such as P–3 and Sn+4, is charged and, consequently, has a name that includes the word "ion." However, an ionic compound, such as Sn3P4, 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, "phosphide ion" is shortened to "phosphide," and "tin (IV) ion" becomes "tin (IV)."
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 phrase "tin (IV)" is written before "phosphide." As the subscripts in an ionic chemical formula are not referenced in an ionic chemical name, the result of combining these terms, "tin (IV) phosphide," is the chemically-correct name for Sn3P4.