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.
Nomenclature of Ionic 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+).
Common Cations on the Periodic Table
|Cation Name||Chemical Symbol||Group in the Periodic Table|
|Sodium ion||Na+||Group 1|
|Magnesium ion||Mg2+||Group 2|
|Iron(II) ion||Fe2+||Group 8 (transition metal)|
|Iron(III) ion||Fe3+||Group 8 (transition metal)|
|Copper(I) ion||Cu+||Group 11 (transition metal)|
|Copper(II) ion||Cu2+||Group 11 (transition metal)|
|Ammonium ion||NH4+||(Polyatomic ion)|
|Hydronium ion||H3O+||(Polyatomic ion)|
Common Anions on the Periodic Table
|Anion Name||Chemical Symbol|
|Anion Name||Chemical Formula|
Examples of Halogen Oxyanions
|Anion Name||Convention||Chemical Formula|
Naming Ionic Compounds
|Magnesium (Mg2+)||Chloride (Cl−)||MgCl2||Magnesium chloride|
|Ammonium (NH4+)||Bromide (Br−)||NH4Br||Ammonium bromide|
|Iron(III) (Fe3+)||Nitrate (NO3−)||Fe(NO3)3||Iron(III) nitrate|
Nomenclature of Binary Covalent Compounds
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.
Greek Prefixes and Their Numbers
Nomenclature of Acids
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.
Examples of Oxyacids
|H3PO4||Phosphate (PO43−)||Phosphoric acid|
|H2SO4||Sulfate (SO42−)||Sulfuric acid|
|H2SO3||Sulfite (SO32−)||Sulfurous acid|
|HClO4||Hypochlorite (ClO4−)||Hypochlorous acid|