In chemical nomenclature, prefixes like mono- or di- are generally not used when naming ionic compounds. Ionic compounds are formed by the transfer of electrons, typically between a metal and a nonmetal, creating positively charged ions (cations) and negatively charged ions (anions). The fundamental reason for omitting prefixes is that the ions must combine in a specific, fixed ratio to ensure the resulting compound is electrically neutral. This means the compound’s formula is automatically determined by the charges of the constituent ions, leaving no ambiguity about the number of ions present.
The Key Distinction in Chemical Naming
The use of numerical prefixes is reserved for naming covalent, or molecular, compounds, which are formed exclusively between nonmetals. Unlike ionic compounds, the atoms in a covalent compound share electrons and can often combine in multiple different whole-number ratios. For instance, carbon and oxygen can form both carbon monoxide (\(\text{CO}\)) and carbon dioxide (\(\text{CO}_2\)), which are chemically distinct substances. Prefixes like mono- (one), di- (two), and tri- (three) are therefore necessary to distinguish between these different possible structures. The name “carbon monoxide” explicitly tells a chemist there is one oxygen atom, while “carbon dioxide” specifies two, which would be impossible to deduce without the prefix. In contrast, the fixed ratio in an ionic compound, such as sodium chloride (\(\text{NaCl}\)), means there is only one correct formula, making prefixes redundant.
Naming Compounds with Fixed-Charge Metal Ions
The simplest ionic compounds involve metals that consistently form only one type of cation, known as fixed-charge metal ions. These include all the elements in Group 1 (alkali metals, which form \(1+\) ions), Group 2 (alkaline earth metals, which form \(2+\) ions), and a few others like aluminum (\(\text{Al}^{3+}\)). The naming convention for these compounds is straightforward and does not require any Roman numerals or prefixes.
The name is constructed by stating the full name of the metal cation first. This is followed by the name of the nonmetal anion, with its ending modified to the suffix “-ide”. For example, the compound \(\text{NaCl}\) is simply called sodium chloride, and \(\text{MgO}\) is named magnesium oxide. The name “calcium bromide” for \(\text{CaBr}_2\) is sufficient because calcium always forms a \(2+\) ion and bromine always forms a \(1-\) ion, which mandates the \(1:2\) ratio in the formula.
Naming Compounds with Variable-Charge Metal Ions
A more complex set of rules is needed for ionic compounds containing metals that can form multiple stable positive charges, referred to as variable-charge metals. This group primarily includes the transition metals, such as iron and copper, but also some others like lead and tin. Since a simple name like “iron chloride” would be ambiguous, the name must clearly specify which ion is present.
To resolve this ambiguity, chemists use the Stock system, which employs Roman numerals in parentheses immediately following the metal’s name. This Roman numeral specifies the exact positive charge, or oxidation state, of the metal ion in that particular compound. For example, iron can form both \(\text{Fe}^{2+}\) and \(\text{Fe}^{3+}\) ions, resulting in two distinct compounds with chlorine: \(\text{FeCl}_2\) and \(\text{FeCl}_3\). These are correctly named iron(II) chloride and iron(III) chloride, respectively. The Roman numeral system replaces the need for numerical prefixes, allowing the reader to deduce the correct ion ratio based on charge balance.
Naming Compounds Containing Polyatomic Ions
Ionic compounds can also be formed with polyatomic ions, which are charged groups of atoms that remain covalently bonded together. Examples include the sulfate ion (\(\text{SO}_4^{2-}\)) and the nitrate ion (\(\text{NO}_3^-\)). Naming these compounds follows the same fundamental ionic rules: the cation name is stated first, followed by the anion name.
The name of the polyatomic ion is simply used in place of the “-ide” suffix. For instance, \(\text{Ca}(\text{NO}_3)_2\) is named calcium nitrate, not calcium dinitrate, even though the formula shows two nitrate ions. The presence of parentheses and a subscript in the chemical formula indicates the number of polyatomic ions needed to achieve charge neutrality, but these numerical details are not translated into prefixes in the compound’s name. Therefore, prefixes are still avoided, maintaining consistency with the general rules for ionic compound nomenclature.