2.7 Chemical Nomenclature of Ionic & Covalent Compounds

As with ionic compounds, the system that chemists have devised for naming covalent compounds enables us to write the molecular formula from the name and vice versa.  In this and the following section, we describe the rules for naming simple covalent compounds.  We begin with inorganic compounds and then turn to simple organic compounds that contain only carbon and hydrogen.

Binary Inorganic Compounds

Binary covalent compounds—that is, covalent compounds that contain only two elements—are named using a procedure similar to that used to name simple ionic compounds, but prefixes are added as needed to indicate the number of atoms of each kind. The procedure, diagrammed in Figure 30, uses the following steps:

Figure 2.30. Naming a Covalent Inorganic Compound

1. Place the elements in their proper order.
   1. The element farthest to the left in the periodic table is usually named first. If both elements are in the same group, the element closer to the bottom of the column is named first.
   2. The second element is named as if it were a monatomic anion in an ionic compound (even though it is not), with the suffix -ide attached to the root of the element name.
2. Identify the number of each type of atom present.  Prefixes derived from Greek stems are used to indicate the number of each type of atom in the formula unit (Table 5).  The prefix mono- (“one”) is used only when absolutely necessary to avoid confusion, just as we omit the subscript 1 when writing molecular formulas.  To demonstrate steps 1 and 2a, we name HCl as hydrogen chloride (because hydrogen is to the left of chlorine in the periodic table) and PCl5 as phosphorus pentachloride. 

The order of the elements in the name of BrF3, bromine trifluoride, is determined by the fact that bromine lies below fluorine in group 17.

Table 5. Prefixes for Indicating the Number of Atoms in Chemical Names

1. If a molecule contains more than one atom of both elements, then prefixes are used for both. Thus N2O3 is dinitrogen trioxide, as shown in.
2. In some names, the final a or o of the prefix is dropped to avoid awkward pronunciation. Thus, OsO4 is osmium tetroxide rather than osmium tetraoxide.
3. Write the name of the compound.

Binary compounds of the elements with oxygen are generally named as “element oxide,” with prefixes that indicate the number of atoms of each element per formula unit.  For example, CO is carbon monoxide.  The only exception is binary compounds of oxygen with fluorine, which are named as oxygen fluorides.  

1. Certain compounds are always called by the common names that were assigned long ago when names rather than formulas were used.  For example, H2O is water (not dihydrogen oxide); NH3 is ammonia; PH3 is phosphine; SiH4 is silane; and B2H6, a dimer of BH3, is diborane.  For many compounds, the systematic name and the common name are both used frequently, so you must be familiar with them.  For example, the systematic name for NO is nitrogen monoxide, but it is much more commonly called nitric oxide. Similarly, N2O is usually called nitrous oxide rather than dinitrogen monoxide.  Notice that the suffixes -ic and -ous are the same ones used for ionic compounds.

Note the Pattern

Start with the element at the far left in the periodic table and work to the right. If two or more elements are in the same group, start with the bottom element and work up.

Example 10

Write the name of each binary covalent compound.

SF6

N2O4

ClO2

Solution

Given: molecular formula

Asked for: name of compound

Strategy:

1.  List the elements in order according to their positions in the periodic table.  Identify the number of each type of atom in the chemical formula and then use Table 2.6 “Prefixes for Indicating the Number of Atoms in Chemical Names” to determine the prefixes needed.
2.  If the compound contains oxygen, follow step 3a. If not, decide whether to use the common name or the systematic name.

Solution:

A.  Because sulfur is to the left of fluorine in the periodic table, sulfur is named first. Because there is only one sulfur atom in the formula, no prefix is needed. B There are, however, six fluorine atoms, so we use the prefix for six: hexa- (Table 2.6 “Prefixes for Indicating the Number of Atoms in Chemical Names”). The compound is sulfur hexafluoride.

B. Because nitrogen is to the left of oxygen in the periodic table, nitrogen is named first. Because more than one atom of each element is present, prefixes are needed to indicate the number of atoms of each. According to Table 2.6 “Prefixes for Indicating the Number of Atoms in Chemical Names”, the prefix for two is di-, and the prefix for four is tetra-. B The compound is dinitrogen tetroxide (omitting the a in tetra- according to step 2c) and is used as a component of some rocket fuels.

C. Although oxygen lies to the left of chlorine in the periodic table, it is not named first because ClO2is an oxide of an element other than fluorine (step 3a). Consequently, chlorine is named first, but a prefix is not necessary because each molecule has only one atom of chlorine. B Because there are two oxygen atoms, the compound is a dioxide. Thus the compound is chlorine dioxide. It is widely used as a substitute for chlorine in municipal water treatment plants because, unlike chlorine, it does not react with organic compounds in water to produce potentially toxic chlorinated compounds.

Example 11

Write the formula for each binary covalent compound.

sulfur trioxide

diiodine pentoxide

Given: name of compound

Asked for: formula

Solution

Strategy:

List the elements in the same order as in the formula to identify the number of each type of atom present, and then indicate this quantity as a subscript to the right of that element when writing the formula.

Solution:

Sulfur has no prefix, which means that each molecule has only one sulfur atom.  The prefix tri- indicates that there are three oxygen atoms. The formula is therefore SO3.  Sulfur trioxide is produced industrially in huge amounts as an intermediate in the synthesis of sulfuric acid.

The prefix di- tells you that each molecule has two iodine atoms, and the prefix penta- indicates that there are five oxygen atoms.  The formula is thus I2O5, a compound used to remove carbon monoxide from air in respirators.

The empirical and molecular formulas discussed in the preceding section are precise and highly informative, but they have some disadvantages.  First, they are inconvenient for routine verbal communication.  For example, saying “C-A-three-P-O-four-two” for Ca3(PO4)2 is much more difficult than saying “calcium phosphate.”  In addition, you will see that many compounds have the same empirical and molecular formulas, but different arrangements of atoms, which result in very different chemical and physical properties.  In such cases, it is necessary for the compounds to have different names that distinguish among the possible arrangements.

Many compounds, particularly those that have been known for a relatively long time, have more than one name: a common name (sometimes more than one) and a systematic name, which is the name assigned by adhering to specific rules.  Like the names of most elements, the common names of chemical compounds generally have historical origins, although they often appear to be unrelated to the compounds of interest.  For example, the systematic name for KNO3 is potassium nitrate, but its common name is saltpeter.

In this text, we use a systematic nomenclature to assign meaningful names to the millions of known substances.  Unfortunately, some chemicals that are widely used in commerce and industry are still known almost exclusively by their common names; in such cases, you must be familiar with the common name as well as the systematic one.  The objective of this and the next two sections is to teach you to write the formula for a simple inorganic compound from its name—and vice versa—and introduce you to some of the more frequently encountered common names.

We begin with binary ionic compounds, which contain only two elements. The procedure for naming such compounds is outlined in Figure 2.10 “Naming an Ionic Compound” and uses the following steps:

Figure 2.31. Naming an Ionic Compound

Ref: commons.wikimedia.org/

1. Place the ions in their proper order: cation and then anion.
2. Name the cation.

1. Metals that form only one cation. As noted before, these metals are usually in groups 1–3, 12, and 13.  The name of the cation of a metal that forms only one cation is the same as the name of the metal (with the word ion added if the cation is by itself).  For example, Na+ is the sodium ion, Ca2+ is the calcium ion, and Al3+ is the aluminum ion.
2. Metals that form more than one cation. As shown before, many metals can form more than one cation.  This behavior is observed for most transition metals, many actinides, and the heaviest elements of groups 13–15.  In such cases, the positive charge on the metal is indicated by a roman numeral in parentheses immediately following the name of the metal.  Thus Cu+ is copper (I) (read as “copper one”), Fe2+ is iron (II), Fe3+ is iron (III), Sn2+ is tin (II), and Sn4+ is tin (IV).

An older system of nomenclature for such cations is still widely used, however.  The name of the cation with the higher charge is formed from the root of the element’s Latin name with the suffix -ic attached, and the name of the cation with the lower charge has the same root with the suffix -ous.  The names of Fe3+, Fe2+, Sn4+, and Sn2+ are therefore ferric, ferrous, stannic, and stannous, respectively.  Even though this text uses the systematic names with roman numerals, you should be able to recognize these common names because they are still often used.  For example, on the label of your dentist’s fluoride rinse, the compound chemists call tin (II) fluoride is usually listed as stannous fluoride.

Some examples of metals that form more than one cation are in table 6, along with the names of the ions.  Note that the simple Hg+ cation does not occur in chemical compounds.  Instead, all compounds of mercury(I) contain a dimeric cation, Hg22+, in which the two Hg atoms are bonded together.

Table 6. Common Cations of Metals That Form More Than One Ion

3. Polyatomic cations. The names of the common polyatomic cations that are relatively important in ionic compounds, such as the ammonium ion.
4. Name the anion.

1. Monatomic anions. Monatomic anions are named by adding the suffix -ide to the root of the name of the parent element; thus, Cl− is chloride, O2− is oxide, P3− is phosphide, N3− is nitride (also called azide), and C4− is carbide.  Because the charges on these ions can be predicted from their position in the periodic table, it is not necessary to specify the charge in the name.  
2. Polyatomic anions. Polyatomic anions typically have common names that you must learn. Polyatomic anions that contain a single metal or nonmetal atom plus one or more oxygen atoms are called oxoanions (or oxyanions).  In cases where only two oxoanions are known for an element, the name of the oxoanion with more oxygen atoms ends in -ate, and the name of the oxoanion with fewer oxygen atoms ends in -ite. For example, NO3− is nitrate and NO2− is nitrite.  The halogens and some of the transition metals form more extensive series of oxoanions with as many as four members.  In the names of these oxoanions, the prefix per- is used to identify the oxoanion with the most oxygen (so that ClO4− is perchlorate and ClO3− is chlorate), and the prefix hypo- is used to identify the anion with the fewest oxygen (ClO2− is chlorite and ClO− is hypochlorite). Differentiating the oxoanions in such a series is no trivial matter.  For example, the hypochlorite ion is the active ingredient in laundry bleach and swimming pool disinfectant, but compounds that contain the perchlorate ion can explode if they come into contact with organic substances.
3. Write the name of the compound as the name of the cation followed by the name of the anion.

It is not necessary to indicate the number of cations or anions present per formula unit in the name of an ionic compound because this information is implied by the charges on the ions.  You must consider the charge of the ions when writing the formula for an ionic compound from its name, however.  Because the charge on the chloride ion is −1 and the charge on the calcium ion is +2, for example, consistent with their positions in the periodic table, simple arithmetic tells you that calcium chloride must contain twice as many chloride ions as calcium ions to maintain electrical neutrality.  Thus, the formula is CaCl2.  Similarly, calcium phosphate must be Ca3(PO4)2 because the cation and the anion have charges of +2 and −3, respectively.  The best way to learn how to name ionic compounds is to work through a few examples.

Figure 2.32. The relationship between the Names of oxoanions and the number of oxygen atoms present

Ref: commons.wikimedia.org/

Note the Pattern

Cations are always named before anions.

Most transition metals, many actinides, and the heaviest elements of groups 13–15 can form more than one cation.

Example 12

Write the systematic name (and the common name if applicable) for each ionic compound.

LiCl

MgSO4

(NH4)3PO4

Cu2O

Solution

Given: empirical formula

Asked for: name

Strategy:

 A. If only one charge is possible for the cation, give its name.  If the cation can have more than one charge, specify the charge using roman numerals.

B. If the anion does not contain oxygen, name it according to step 3a.  For polyatomic anions that contain oxygen, use the appropriate prefix and suffix listed in step 3b.

C. Beginning with the cation, write the name of the compound.

Solution:

A. Lithium is in group 1, so we know that it forms only the Li+ cation, which is the lithium ion.  Similarly, chlorine is in group 7, so it forms the Cl− anion, which is the chloride ion.  Because we begin with the name of the cation, the name of this compound is lithium chloride, which is used medically as an antidepressant drug.

B. The cation is the magnesium ion, and the anion, which contains oxygen, is sulfate.  Because we list the cation first, the name of this compound is magnesium sulfate.  A hydrated form of magnesium sulfate (MgSO4·7H2O) is sold in drugstores as Epsom salts, a harsh but effective laxative.

C. The cation is the ammonium ion and the anion is phosphate.  The compound is therefore ammonium phosphate, which is widely used as a fertilizer.  It is not necessary to specify that the formula unit contains three ammonium ions because three are required to balance the negative charge on phosphate.

D. The cation is a transition metal that often forms more than one cation. We must therefore specify the positive charge on the cation in the name: copper (I) or, according to the older system, cuprous.  The anion is oxide.  The name of this compound is copper (I) oxide or, in the older system, cuprous oxide.  Copper (I) oxide is used as a red glaze on ceramics and in antifouling paints to prevent organisms from growing on the bottoms of boats. Cu2O. The bottom of a boat is protected with a red antifouling paint containing copper (I) oxide, Cu2O.