Nomenclature for Inorganic Compounds

Chemical nomenclature is the process of naming chemical substances based on a set system of rules as defined by the International Union of Pure and Applied Chemistry (IUPAC). These rules were constructed by scientists as a standard to identify chemicals worldwide using the same names, regardless of language. Chemical nomenclature is maintained and updated periodically by IUPAC. Some substances that are commonly used in everyday life, such as water or baking soda, are often referred to by their common names. Most chemical substances do not have widely used common names. Having a set of rules for naming these substances can help prevent confusion.

Chemistry is often divided into two broad categories: organic compounds and inorganic compounds. An organic compound consists of molecules that contain one or more carbon-hydrogen bonds. There is an enormous variety of organic compounds, and their nomenclature is detailed to reflect that. An inorganic compound consists of molecules that do not contain a carbon-hydrogen bond. Inorganic compounds are often studied as ionic compounds, molecular compounds, and acids. This section will focus on the nomenclature of inorganic compounds.

Ionic compounds, a compound where metals and nonmetals are bonded together, are made of ions. Most ionic compounds are solids and form extended structures in which each ion is bonded to a group of other ions. An ion can be a single element or have a polyatomic structure that includes more than one atom. A positive ion is called a cation, and a negative ion is called an anion. Ionic compounds are formed between one or more cations and one or more anions. Metals generally have low electronegativity and tend to lose electrons, forming a cation. Nonmetals generally have a high electronegativity, and they tend to gain electrons, forming an anion. An ionic compound typically consists of a metal cation bonded to a nonmetal anion.

To name ionic compounds, first cations and anions should be named. A single-atom cation is named after its parent element. For example, the potassium ion (K⁺) is named after the potassium element (K). Group 1 and group 2 metals always form cations with 1+ and 2+ charges, respectively. Transition metals may form cations with several different charges. In the case of transition metals, the charge of the ion is written as a Roman numeral in parentheses after the element name. For example, a mercury(II) ion (Hg²⁺) has a 2+ charge. The charge of an ion is the sum of the oxidation states of all the atoms in the ion. For ions that are only one atom, the charge and the oxidation state are equal. Two of the many polyatomic ions are the ammonium ion (NH₄⁺) and the hydronium ion (H₃O⁺).

Anions are named differently than cations. A single-atom anion is named by adding the suffix -ide at the end of the name of its parent element. Thus, the ion of the element fluorine is named fluoride, and the ion of oxygen is named oxide. Some polyatomic ions also end in with the suffix -ide.

Polyatomic anions are common in inorganic chemistry. An oxyanion is a polyatomic anion that contains oxygen along with another element. The same element may form more than one oxyanion with oxygen. The most common oxyanion of an element has the suffix -ate. If there is a second oxyanion with fewer oxygen atoms but the same charge, that oxyanion has the suffix -ite.

Oxyanions of halogens (group 17 elements) are more complicated. Halogens can form four different oxyanions. The oxyanion with the most oxygen atoms has the prefix per- and the suffix -ate. The oxyanion with the second-most oxygen atoms has the suffix -ate. The oxyanion with the third-most oxygen atoms has the suffix -ite. The oxyanion with the fewest oxygen atoms has the prefix hypo- and the suffix -ite.

Many oxyanions can form another anion by adding one or more hydrogen atoms. In this case the word hydrogen is added in front of the ion. Hydrogen may get the prefix di- if there are two hydrogen atoms in the anion. Note that hydrogen takes the oxidation state +1 in oxyanions. Adding hydrogen to an oxyanion will increase the overall charge of the ion by one per hydrogen Examples of hydrogen oxyanions are hydrogen carbonate ion (HCO₃⁻) and dihydrogen phosphate ion (H₂PO₄⁻).

Ionic compounds consisting of just two ions are named by the cation followed by the name of anion. The compound NaCl, for example, becomes sodium chloride. If the element forming the cation can form more than one cation, the exact cation is specified using parenthesis. A Roman numeral added in parentheses immediately after the cation indicates its charge. For example, the compound Cr₂O₃ is chromium(III) oxide. The relative ratios of cations and anions are disregarded when naming ionic compounds. The compound CaCl₂ is simply calcium chloride.

Covalent, or molecular, compounds commonly form between two nonmetals or between a nonmetal and a metalloid. A binary compound is a compound made of two elements. Binary covalent compounds are named similarly to ionic compounds. Follow these steps when naming binary covalent compounds:

1. Write the name of the element that is farther left on the periodic table first. In hydrogen sulfide (H₂S), for example, hydrogen is written first because it is found on the left side of the periodic table.

• The exception to this is when oxygen bonds with chlorine, bromine, or iodine (all halogens except fluorine). In this case, oxygen is written last. The molecule Cl₂O is dichlorine monoxide.
• Note, however, that when oxygen bonds with fluorine, oxygen is written first. The compound OFl₂ is oxygen difluoride.

2. If the elements are in the same group, the element lower in the column is written first. For example, sulfur and oxygen are both group 16 elements. In the compound sulfur dioxide (SO₂), sulfur is written first because of this rule.

3. Add the suffix -ide to the name of the second element.

4. Use Greek prefixes to describe how many of each atom type is in the molecule. Examples include carbon dioxide (CO₂), nitrogen triiodide (NI₃), and tetraphosphorus decasulfide (P₄S₁₀).

• An exception to this rule is that the prefix mono- is not used for the first element in most molecules.

An acid, according to the Arrhenius definition of an acid (named for Swedish chemist Svante Arrhenius), is a substance that releases hydrogen (H⁺) ions when dissolved in water. An acid is composed of an anion and enough hydrogen ions to neutralize the negative charge of the anion. Thus, the acid based on the phosphate ion (PO₄³⁻) has three hydrogen ions because of the 3– charge of the phosphate ion.

A binary acid is an acid made of hydrogen and one other element. An example of a binary acid is hydrochloric acid (HCl). In a binary acid, the anion is a single element and ends with the -ide suffix. To name the acid, replace the -ide suffix with an -ic suffix, add a hydro- prefix in front of the anion, and follow with the word acid. For example, the acid consisting of a hydrogen ion and a bromide ion is hydrobromic acid (HBr). Other examples of binary acids are hydrofluoric acid (HF) and hydrosulfuric acid (H₂S).

An oxyacid is an acid that contains oxygen. Oxyacids end with either the suffix -ate or the suffix -ite. Follow the rules below when naming oxyacids:

• If the anion ends with the suffix -ate, replace the suffix with -ic, and follow with the word acid. For example, the acid containing the nitrate ion becomes nitric acid.
• If the anion ends with the suffix -ite, replace the suffix with -ous, and follow with the word acid. For example, the acid containing the nitrite ion becomes nitrous acid.
• If the oxyanion has any prefixes, such as per- or hydro-, keep them in the name. For example, the acid containing perchlorate ion becomes perchloric acid.