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. 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
Monatomic cations bear the same names as their elements, with the addition of the word 'ion'. Many elements (such as sodium and calcium) have only one stable form of cation in solution. Hence, Na+ is called the sodium ion, and Ca2+ is called the calcium ion. Na2+ and Ca+ ions are not stable in solutions. Notice that if you refer to the periodic chart, with no exception, the stable ion of all the Group IA metals (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 about 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 of the cations that have only one stable form are listed in Table 1.)
Some metals, especially the transition metals (with a few exceptions that are printed in blue in Table I), 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 system has different suffixes in their names. For example, Fe2+ is called the ferrous ion, and Fe3+ is called the ferric ion; Cu+ is the cuprous ion, and 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. In contrast, 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
Monatomic 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 to the left of the noble gases in the periodic chart. The oxide and sulfide ions carry a -2 charge because they are located 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 (shown in blue) 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 that 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; hence, they often form similar polyatomic ions. Therefore, if we know the name and formula for a particular polyatomic ion, then by analogy, we can determine the name and formula of the similar polyatomic ion of another element 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–.
It is important to know the names of polyatomic ions, and it is equally important to be familiar with their size and shape. Click “ Molecular Representations” to see 2D and 3D representations of some of the ions from table IV and to learn about how molecules are often represented in chemistry and biology.
Naming CompoundsFor the purpose of nomenclature, the inorganic compounds can be separated into 4 categories.
I. Compounds of high ionic character ----Two types of compounds fall into this category: those consisting of a metal combined with a nonmetal (e.g., NaCl, Ag2S, PbO) and those containing polyatomic ions, except for the oxoacids (e.g., CaSO4, NH4NO3, KCN, but excluding H2SO4, HNO3, etc.). For the sake of naming compounds, both of these categories will be classified as ionic compounds in this tutorial.
To name an ionic compound, one should name the cation first, and then name the anion (with the word 'ion' omitted). It is not necessary to indicate the number of cations and anions in the compound because it is understood that the total positive charges carried by the cations must equal the total negative charges carried by the anions. A few examples are listed below:
Some ionic compounds incorporate water molecules in their structure. These compounds are called hydrates. To name the hydrates, the number of waters of hydration is indicated by a Greek prefix following the name of the compound. For example, CuSO4·5H2O is called copper(II) sulfate pentahydrate.
Determining the molecular formula from the compound's name is not always straightforward. This is because the number of cations and anions in a molecule is not specified in the name of an ionic compound. The following examples show how finding the molecular formula can be achieved in a systematic matter:
Example 1. Give the molecular formula of aluminum sulfide.
i)Since aluminum is a metal and sulfur is a nonmetal, this compound is classified as an ionic compound.
ii)The cation, aluminum ion, is: Al3+ (if you forget the charge of the aluminum ion, look up the position of Al in the periodic chart).
iii)The anion, sulfide, is: S2- (the –ide suffix indicates that it is a simple anion).
iv)How many Al3+ ions should combine with the appropriate number of S2- ions such that the molecule carries no net charge? Al2S3 is the answer.
Example 2. Give the molecular formula of vanadium(III) phosphate.
i)You may not recognize that vanadium is a metal. However, the suffix –ate in the word 'phosphate' is the hint of an oxoanion, a polyatomic ion. You know that this compound is classified as an ionic compound.
ii)The cation is vanadium(III) = V3+.
iii)The anion is phosphate = PO43-.
iv)How many V3+ ions should combine with the appropriate number of PO43- ions such that the molecule carries no net charge? VPO4 is the answer.
Example 3. Give the molecular formula of ammonium sulfate.
i)Both ammonium and sulfate are polyatomic ions. Again, this compound is classified as an ionic compound.
ii)The cation is ammonium ion = NH4+.
iii)The anion is sulfate ion = SO42-.
iv)The molecular formula of the compound is (NH4)2SO4 because it takes two NH4+ ions to combine with one SO42- ion to give a molecule that carries no charge.
(click here to see the answers)
1. Name the following ionic compounds:
2. Give the chemical formulas for the following ionic compounds:
magnesium dihydrogen phosphate
ammonium ferrous sulfate hexahydrate
II. Compounds of high covalent character ---- Compounds consisting of only nonmetals and no polyatomic ions belong to this category (e.g., SO2, NH3, CS2 but not NH4Cl because NH4+ is a polyatomic cation). 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.
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:
3. Name the following covalent compounds:
4. Give the chemical formulas for the following covalent compounds:
III. Inorganic acids ---- The rules used to name inorganic acids are different from those rules used to name the ionic and 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+ ions bonded to an SO42- ion). 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,
HNO3 is H+ bonded to NO3– (nitrate), thus it is called nitric acid.
HNO2 is H+ bonded to NO2– (nitrite), thus it is called nitrous acid.
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 among the names of the anions and the names of the acids are 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.
5. Name the following compounds/ions:
6. Give the chemical formulas for the following compounds/ions:
ammonium hydrogen phosphate
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 discussed in this course in the near future. To learn more about this topic, please click “ Naming Coordination Compounds”.
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 that 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:
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:
The author thanks Regina Frey and Amy Walker for many helpful suggestions in the writing of this tutorial.
Updated On: 7/8/2011