An anion is an ion that possesses a net negative electrical charge, acquired when a neutral atom or molecule gains one or more electrons. Chemical nomenclature provides a systematic set of rules for naming these charged species. The method used depends primarily on the anion’s structure: whether it is composed of a single atom or multiple atoms. Understanding these rules is fundamental to correctly identifying chemical compounds.
Naming Single-Element Anions (Monoatomic)
The most straightforward naming convention applies to monoatomic anions, which are ions formed from a single non-metal atom. These elements, typically found on the right side of the periodic table, readily gain electrons to achieve a stable electron configuration. The systematic rule for naming these ions involves altering the ending of the element’s name.
The process requires dropping the original last syllable or ending of the element’s name and replacing it with the suffix “-ide.” For example, when a neutral chlorine atom gains an electron, it becomes the chloride ion. Similarly, a single oxygen atom gaining two electrons is systematically named oxide.
This rule applies consistently across many common elements. Sulfur, upon gaining two electrons, is named sulfide, while nitrogen becomes nitride when it gains three electrons. The “-ide” suffix signals that the ion is a monoatomic species with a negative charge.
The Core System for Polyatomic Oxyanions (-ate and -ite)
Naming becomes more complex when dealing with polyatomic anions, which are negatively charged ions composed of two or more atoms chemically bonded together. A significant subgroup is the oxyanions, which are polyatomic ions containing oxygen atoms bonded to a central non-metal atom. The core naming system for these ions is based on the relative number of oxygen atoms present in the structure.
When a central element, such as sulfur or nitrogen, forms two different oxyanions, a comparative system is used to distinguish them. The ion containing the greater number of oxygen atoms is designated with the suffix “-ate.” The ion with the fewer number of oxygen atoms is given the suffix “-ite.”
This difference in suffix provides chemists with immediate information about the ion’s composition. For instance, the sulfate ion (\(\text{SO}_4^{2-}\)) contains four oxygen atoms, whereas the sulfite ion (\(\text{SO}_3^{2-}\)) contains only three, yet both have a sulfur atom at their core. In the nitrogen system, the nitrate ion (\(\text{NO}_3^-\)) has three oxygen atoms, while the nitrite ion (\(\text{NO}_2^-\)) has two.
The root name of the central element, like “sulf” or “nitr,” remains consistent between the two ions, simplifying the identification process. This systematic contrast between the “-ate” and “-ite” suffixes establishes a foundational convention for many polyatomic ions.
Expanding the Oxyanion Series (Using Hypo- and Per- Prefixes)
Some non-metal elements can form more than two distinct oxyanions, requiring an expansion of the core naming system to distinguish all four species. When this occurs, two prefixes are introduced to modify the established “-ite” and “-ate” suffixes, creating a four-member series. This expanded system uses the prefixes “hypo-” and “per-” to denote the extremes of oxygen content.
The prefix “hypo-” is used for the oxyanion that contains the fewest oxygen atoms. This prefix is always paired with the “-ite” suffix, resulting in the name “hypo\_\_\_ite.” Conversely, the prefix “per-” is reserved for the oxyanion containing the greatest number of oxygen atoms. The “per-” prefix is always paired with the “-ate” suffix, resulting in the name “per\_\_\_ate.”
The chlorine oxyanion series provides the clearest example of this expanded nomenclature. The four ions are hypochlorite (\(\text{ClO}^-\)), chlorite (\(\text{ClO}_2^-\)), chlorate (\(\text{ClO}_3^-\)), and perchlorate (\(\text{ClO}_4^-\)). This sequential application of prefixes and suffixes allows for the precise naming of each ion based solely on its oxygen count. The names move from “hypo-ite” (lowest oxygen) through “ite” and “ate,” up to “per-ate” (highest oxygen).
Anions Containing Hydrogen
A unique class of anions includes those that still retain one or more hydrogen atoms within their structure, often referred to as acid anions. These ions are produced when a polyprotic acid, which is an acid capable of donating more than one proton, only partially dissociates in solution. The remaining hydrogen atom is still attached to the anion, contributing to its overall structure and charge.
The systematic naming convention for these species is straightforward and involves simply adding the prefix “hydrogen” to the name of the parent anion. For example, the parent ion carbonate (\(\text{CO}_3^{2-}\)) can partially neutralize to form the hydrogen carbonate ion (\(\text{HCO}_3^-\)). Similarly, the phosphate ion (\(\text{PO}_4^{3-}\)) can form the hydrogen phosphate (\(\text{HPO}_4^{2-}\)) and dihydrogen phosphate (\(\text{H}_2\text{PO}_4^-\)) ions.
When two hydrogen atoms are retained, the prefix “dihydrogen” is used to specify this count, as seen in the dihydrogen phosphate ion. While the systematic “hydrogen” nomenclature is the current standard, many of these ions have older, non-systematic common names that persist in use. The hydrogen carbonate ion, for instance, is still commonly called bicarbonate.
Using the “hydrogen” prefix provides a clear and systematic method for communicating the exact composition of these partially neutralized anions. The older “bi-” prefix, such as in “bisulfate” for hydrogen sulfate, is largely considered obsolete in formal chemical nomenclature.