What's in a Name?

The Nomenclature of Inorganic Compounds

Author: Kit Mao
Department of Chemistry
Washington University,
St. Louis, MO 63130

       

Key Concepts


The Cautionary Tale of Dihydrogen Monoxide…

The following is an excerpt from an article in Natural History 5/98 by Neil de Grasse Tyson:

Nathan Zohner, a student at Eagle Rock Junior High School in Idaho, conducted a clever science-fair experiment that tested anti-technology sentiments and associated chemical phobias in 1997. He invited people to sign a petition that demanded either strict control of, or a total ban on, dihydrogen monoxide. He listed some of the odious properties of this colorless and odorless substance:

Forty-three out of fifty people approached by Zohner signed the petition, six were undecided, and one was a strong supporter of dihydrogen monoxide and refused to sign. Yes, 86% of the passersby voted to ban water (H2O) from the environment.

We live in a world made of chemicals. Table salt is sodium chloride; sugar is a disaccharide; a major ingredient of vinegar is acetic acid; glass is a super-cooled liquid silicate; our stomach contains 1 M hydrochloric acid. So, as you can see it is important to be able to recognize a chemical by its name. In this tutorial, you will learn about the systematic naming of inorganic compounds.

Naming Simple Cations

The monoatomic cations bear the same names of the elements with the addition of the word ‘ion’. For example, Na+ is called the sodium ion and Ca2+ is called the calcium ion. What do you call a Na2+ ion, or a Ca+ ion? Well, Na2+ and Ca+ are not stable ions in solids or solutions. Therefore, whenever one mentions a sodium ion, we know that it is always Na+ and a calcium ion is always Ca2+. Some of the cations that have only one stable form are listed in Table I. Notice that if you refer to the periodic chart, with no exception, all the Group IA metals (they are called alkali metals) have a +1 charge and the Group IIA metals (alkaline earth metals) have a +2 charge. This is due to the ground-state electron configurations of these elements, a topic you will learn in the Chem 111A lectures in the near future. Other common metal cations that have only one stable oxidation state are: Al3+, Ga3+, Ni2+, Zn2+, Cd2+, and Ag+.

Some metals, especially the transition metals, can form more than one type of cation, such as Fe2+ and Fe3+ or Cu+ and Cu2+. To distinguish between these ions, there are two naming systems. The old style system has different suffixes in their names, e.g. Fe2+ is called the ferrous ion, Fe3+ is called the ferric ion and Cu+ is the cuprous ion, Cu2+ is the cupric ion. Notice that the ion with the lesser charge ends with –ous and the one with greater charges ends with –ic. The systematic naming method used today indicates the charge of the ion with a Roman numeral in parentheses (called the Stock number) immediately following the ion’s name. Thus, Fe2+ is an iron(II) ion and Pb4+ is a lead(IV) ion. Ca2+ is just calcium ion, not calcium(II) ion, because calcium only has one kind of stable cation. The names of some simple cations are listed in Table II.

* Despite the +2 charges, each Hg in the Hg22+ ion only carries a charge of +1 (the oxidation number is +1). This is why it is called mercury(I) ion.

Naming Simple Anions

The monoatomic anions are named by adding the suffix -ide to the stem of the name of the nonmetallic elements from which the anion is derived. For example, Cl- is called chloride and S2- is called sulfide. Like a cation, the charge carried by an anion is related to the ground state electron configuration of the element and thus is related to the position of the element in the periodic chart. All the halogen anions (they are called halide ions) carry a - 1 charge because the halogen group is one group left of the noble gases in the periodic chart. The oxide and sulfide carries a - 2 charge because they are two groups away from the noble gases in the periodic chart. Following this logic, one can predict that the nitride ion and the phosphide ion must carry a - 3 charge. Some of the simple anions and their names are listed in Table III. The hydride, peroxide, superoxide, and carbide ions are exceptions to the above rule.

Naming Polyatomic Ions

Some of the names and charges of common polyatomic cations and anions are listed in Table IV.

Notice that there are a lot more polyatomic anions than cations. Most polyatomic anions, consist of a nonmetallic element combined with different numbers of oxygen atoms (these polyatomic anions are called oxoanions.) Even though it seems there is no simple rule in naming these ions, in fact, here are some guidelines to follow:

It should be noted that the acetate and oxalate ions (in purple) are organic ions. They follow the naming system of organic compounds. They are included for reference here, as they are commonly used in Chem 111A, 112A, 151 and 152.

Elements in the same group of the periodic chart have similar chemical properties. Very often, they form similar polyatomic ions. One can draw analogy in the names and formulas of the polyatomic ions of elements in the same group. For example, if one knows that chlorate ion is ClO3- , then, an educated guess for the formula of bromate ion is BrO3- and for iodate ion is IO3- .

Naming Compounds

For the purpose of nomenclature, the inorganic compounds can be separated into 4 categories.


Practice Problems (the answer key is located at the last page of this tutorial)

1. Name the following ionic compounds:

2. Give the chemical formulas for the following ionic compounds:


II. Compounds of great covalent character---- Compounds consisting of only nonmetals and no polyatomic ions belong to this category (e.g. SO2, NH3, CS2 but excluding NH4Cl). They will be called covalent compounds in this tutorial.

To name the covalent compounds, name the electropositive (or less electronegative) element first, then, name the more electronegative element as if the more electronegative element is a simple anion (ending with - ide). How does one know which element is the electropositive element? In the chemical formulas of covalent compounds, usually the symbol of the electropositive element precedes the more electronegative element (e.g. SO2, CO, and SF6. NH3 is an exception of this generalization.). If one follows this rule, then, SO2 would be called sulfur oxide and CO would be called carbon oxide. Very often, two nonmetals can combine to form more than one compound. For example, carbon and oxygen can combine to form CO2 or CO; sulfur and oxygen can combine to form SO2 or SO3. To distinguish these compounds from each other, Greek prefixes are used to designate the numbers of atoms of one or both elements in the molecule. Therefore, CO2 is called carbon dioxide and CO is called carbon monoxide; SO2 is sulfur dioxide and SO3 is sulfur trioxide).
Greek prefixes: mono- 1 hexa-6
  di-2 hepta-7
  tri-3 octa-8
  tetra-4 nona-9
  penta-5 deca-10

The following are a few examples:
NF3 nitrogen trifluoride
N2O4 dinitrogen tetraoxide
OF2 oxygen difluoride

For historical reasons, some hydrogen containing covalent compounds have nonsystematic names. Such as:

H2O water
NH3 ammonia
PH3 phosphine;
N2H4 hydrazine
SiH4 silane

Practice Problems (the answer key is located at the last page of this tutorial)

3. Name the following covalent compounds:

4. Give the chemical formulas for the following covalent compounds:


III. Inorganic acids ---- The naming of inorganic acids does not follow the rules of the ionic compounds or covalent compounds. For example, HNO3 is called nitric acid, not hydrogen nitrate nor hydrogen nitrogen trioxide. How can one recognize an acid by looking at its chemical formula? You will learn about the properties of acids in detail in the second semester of general chemistry. Here we will simply present the rules for naming acids. An acid is a proton donor. Therefore, for the purpose of nomenclature, an acid can be viewed as a molecule with one or more protons (H+) bonded to an anion. Note that the molecule must not carry a charge. For example, HSO3- is not an acid molecule; it is an anion because it carries a - 1 charge. Even though it shows acidic properties, it is named like a polyatomic anion. Also, the molecule must not contain metal atoms. For example, NaHSO3, should not be named as an acid. Instead, it should be named as an ionic compound because it consists of a Na+ cation and an HSO3- anion. Thus, it is named sodium bisulfite or sodium hydrogen sulfite.

Many acids consist of protons bonded to an oxoanion (e.g. HNO3 is H+ bonded to NO3- and H2SO4 is two H+s bonded to a SO42- ). These acids are called oxoacids. To name an oxoacid, one should change the - ate or - ite suffixes of the oxoanions to - ic or - ous respectively and add the word acid at the end. For example,

Besides the oxoacids, there are other acids in which the anions end with the suffix - ide. The names of these acids begin with hydro- and end with - ic. For example, aqueous HCl is called hydrochloric acid because the anion, Cl- , is named chloride.

The names of the inorganic acids are closely related to the names of the anions in the acid. The correlations of the names of the anions and the names of the acids is summarized in Table V below with examples:

Note: The gaseous HCl, HBr, H2S etc. do not bear the names of acids. They are named as covalent compounds. A compound that dissolves in water to form an acid is called an acid anhydride (acid without water). Only the aqueous solutions of acid anhydrides are named as acids. Therefore, HCl(g) is called hydrogen chloride while HCl(aq) is called hydrochloric acid; HCN(g) is called hydrogen cyanide while HCN(aq) is called hydrocyanic acid. The distinction in naming the anhydrides and the acids is not critical for oxoacids, because all their anhydrides are different molecules. For example, the anhydride of H2SO4 is SO3, not gaseous H2SO4. Thus H2SO4 is always called sulfuric acid, not hydrogen sulfate.

Practice Problems (the answer key is located on the last page of this tutorial)

5. Name the following compounds/ions:

6. Give the chemical formulas for the following compounds/ions:


IV. Coordination compounds---- This family of compounds consists of central metal ion(s) bonded to molecules or anions called ligands. The nomenclature of these compounds will be excluded from this tutorial. It will be discussed in this course in the near future.

Summary

Knowing the symbols and charges of the cations and anions is essential for the nomenclature of inorganic compounds. For the monoatomic ions, you can figure out the charges from the position of the element in the periodic chart. If the element is a transition metal which typically has more than one stable oxidation state, very often, the charge on the ion is indicated by the stock number (several exceptions such as Zn2+, Cd2+ and Ag+). For the polyatomic ions, one must spend more effort to get familiar with their formulas and charges.

The most important strategy in naming a chemical (or in predicting the formula from a given name) is to put it into the correct category. The following flow chart can help you categorize a chemical:


Additional Links:

Tables of Common Polyatomic Ions provides 2-dimensional structures and 3-dimensional representations of some common polyatomic ions.

Naming of Coordination Compounds summarizes the steps and provides examples of naming compounds containing coordination complexes.

Acknowledgements:

The author thanks Regina Frey and Amy Walker for many helpful suggestions in the writing of this tutorial.


Answer to the Practice Problems

1. Name the following ionic compounds:

2. Give the chemical formulas for the following ionic compounds:

3. Name the following covalent compounds:

4. Give the chemical formulas for the following covalent compounds:

5. Name the following compounds/ions:

6. Give the chemical formulas for the following compounds/ions:


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This page was updated 9/5/08