Atoms, Ions, and Molecules

Nomenclature for Inorganic Compounds

Established naming conventions can be used to name ionic and molecular 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.

Nomenclature of Ionic Compounds

Ionic compounds are typically metals bonded with nonmetals, and the cation and anion that form the compound provide the basis for their nomenclature.

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)

A cation is an ion with fewer electrons than protons, giving it a positive charge. Some elements can form two or more types of cations with different electrical charges.

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.

Common Anions on the Periodic Table

Anion Name Chemical Symbol
Chloride ion Cl
Oxide ion O2−
Sulfide ion S2−
Hydroxide ion OH
Peroxide ion O22−

A single-atom anion is named by adding the suffix -ide at the end of the name of its parent element.

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.

Common Oxyanions

Anion Name Chemical Formula
Nitrate ion NO3
Nitrite ion NO2
Sulfate ion SO42−
Sulfite ion SO32−
Carbonate ion CO32−

Oxyanions are polyatomic anions that contain oxygen along with another element.

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.

Examples of Halogen Oxyanions

Anion Name Convention Chemical Formula
Perchlorate ion per-chlorine-ate ClO4
Chlorate ion chlorine-ate ClO3
Chlorite ion chlorine-ite ClO2
Hypochlorite ion hypo-chlorine-ite ClO

The oxyanion with the most oxygen atoms has the prefix per- and the suffix -ate, and the oxyanion with the third-most oxygen atoms has 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).

Naming Ionic Compounds

Ionic compounds are those with ionic bonds and are named according to the identity of the cation.
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.

Ionic Compounds

Cation Anion Compound Name
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

Ionic compounds consisting of just two ions are named by the name of the cation followed by the name of the anion.

Nomenclature of Binary Covalent Compounds

Binary covalent molecules are named similarly to ionic compounds, with the addition of Greek prefixes indicating the number of atoms.

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

Prefix Number
Mono- 1
Di- 2
Tri- 3
Tetra- 4
Penta- 5
Hexa- 6
Hepta- 7
Octa- 8
Nona- 9
Deca- 10

Greek prefixes are used to describe how many of each atom type are in the molecule. An exception to this rule is that the prefix mono- is not used for the first element in most molecules

Nomenclature of Acids

An acid releases hydrogen ions (H+) when dissolved in water. An acid consists of an anion bonded to hydrogen, with its naming conventions following naming conventions of anions.

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

Chemical Formula Oxyanion Name
H3PO4 Phosphate (PO43−) Phosphoric acid
H2SO4 Sulfate (SO42−) Sulfuric acid
H2SO3 Sulfite (SO32−) Sulfurous acid
HClO4 Hypochlorite (ClO4) Hypochlorous acid

Oxyanions that form oxyacids end with either the suffix -ate or the suffix -ite.