4.5: Formulas and Names of Binary Ionic Compounds

Learning Objectives

• Use the rules for naming ionic compounds.
• Write the correct formula for an ionic compound.

Names of Binary ionic Compounds

After learning a few more details about the names of individual ions, you will be a step away from knowing how to name ionic compounds. This section begins the formal study of nomenclature, the systematic naming of chemical compounds.

Naming Ions

The name of a monatomic cation is simply the name of the element followed by the word ion. Thus, Na⁺ is the sodium ion, Al³⁺ is the aluminum ion, Ca²⁺ is the calcium ion, and so forth.

We have seen that some elements lose different numbers of electrons, producing ions of different charges (Figure 4.4.1) . Iron, for example, can form two cations, each of which, when combined with the same anion, makes a different compound with unique physical and chemical properties. Thus, we need a different name for each iron ion to distinguish Fe²⁺ from Fe³⁺. The same issue arises for other ions with more than one possible charge.

There are two ways to make this distinction. In the simpler, more modern approach, called the Stock system (Table \(\PageIndex{1}\)), an ion’s positive charge is indicated by a roman numeral in parentheses after the element name, followed by the word ion. Thus, Fe²⁺ is called the iron(II) ion, while Fe³⁺ is called the iron(III) ion. This system is used only for elements that form more than one common positive ion. We do not call the Na⁺ ion the sodium(I) ion because (I) is unnecessary. Sodium forms only a 1+ ion, so there is no ambiguity about the name sodium ion.

Table \(\PageIndex{1}\): The Modern and Common System of Cation Names

Element	Stem	Charge	Modern Name (Stock System)	Common Name
iron	ferr-	2+	iron(II) ion	ferrous ion
		3+	iron(III) ion	ferric ion
copper	cupr-	1+	copper(I) ion	cuprous ion
		2+	copper(II) ion	cupric ion
tin	stann-	2+	tin(II) ion	stannous ion
		4+	tin(IV) ion	stannic ion
lead	plumb-	2+	lead(II) ion	plumbous ion
		4+	lead(IV) ion	plumbic ion
chromium	chrom-	2+	chromium(II) ion	chromous ion
		3+	chromium(III) ion	chromic ion
gold	aur-	1+	gold(I) ion	aurous ion
		3+	gold(III) ion	auric ion

The second system, called the common system, is not conventional but is still prevalent and used in the health sciences. This system recognizes that many metals have two common cations. The common system uses two suffixes (-ic and -ous) that are appended to the stem of the element name. The -ic suffix represents the greater of the two cation charges, and the -ous suffix represents the lower one. In many cases, the stem of the element name comes from the Latin name of the element. Table \(\PageIndex{1}\) lists the elements that use the common system, along with their respective cation names.

Table \(\PageIndex{2}\): Some Monatomic Anions

Ion	Name
F^−	fluoride ion
Cl^−	chloride ion
Br^−	bromide ion
I^−	iodide ion
O^2−	oxide ion
S^2−	sulfide ion
P^3−	phosphide ion
N^3−	nitride ion

The name of a monatomic anion consists of the stem of the element name, the suffix -ide, and then the word ion. Thus, as we have already seen, Cl⁻ is “chlor-” + “-ide ion,” or the chloride ion. Similarly, O²⁻ is the oxide ion, Se²⁻ is the selenide ion, and so forth. Table \(\PageIndex{2}\) lists the names of some common monatomic ions.

Example \(\PageIndex{1}\)

Name each ion.

1. Ca²⁺
2. S²⁻
3. Cu⁺

Solution

1. the calcium ion
2. the sulfide ion
3. the copper(I) ion or the cuprous ion

Exercise \(\PageIndex{1}\)

Name each ion.

1. Fe²⁺
2. Fe³⁺
3. Ba²⁺

Answer a:

iron(II) ion

Answer b:

iron(III) ion

Answer c:

barium ion

Example \(\PageIndex{2}\)

Write the formula for each ion.

1. the bromide ion
2. the cupric ion
3. the magnesium ion

Solution

1. Br⁻
2. Cu²⁺
3. Mg²⁺

Exercise \(\PageIndex{2}\)

Write the formula for each ion.

1. the fluoride ion
2. the stannous ion
3. the potassium ion

Answer a:

F^-

Answer b:

Sn ²⁺

Answer c:

K⁺

Naming Binary Ionic Compounds with a Metal that Forms Only One Type of Cation

Now that we know how to name ions, we are ready to name ionic compounds. A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. The metal cation is named first, followed by the nonmetal anion as illustrated in Figure \(\PageIndex{1}\) for the compound BaCl₂. The word ion is dropped from both parts.

Figure \(\PageIndex{1}\) Naming \(BaCl_2\)

Naming formula: Name of metal cation + base name of nonmetal anion + suffix -ide. BaCl2 is named as barium chloride.

Subscripts in the formula do not affect the name.

Example \(\PageIndex{3}\): Naming Ionic Compounds

Name each ionic compound.

1. CaCl₂
2. AlF₃
3. KCl

Solution

1. Using the names of the ions, this ionic compound is named calcium chloride.
2. The name of this ionic compound is aluminum fluoride.
3. The name of this ionic compound is potassium chloride

Exercise \(\PageIndex{3}\)

Name each ionic compound.

1. AgI
2. MgO
3. Ca₃P₂

Answer a:

silver iodide

Answer b:

magnesium oxide

Answer c:

calcium phosphide

Naming Binary Ionic Compounds with a Metal That Forms More Than One Type of Cation

If you are given a formula for an ionic compound whose cation can have more than one possible charge, you must first determine the charge on the cation before identifying its correct name. For example, consider FeCl₂ and FeCl_{3 .} In the first compound, the iron ion has a 2+ charge because there are two Cl⁻ ions in the formula (1− charge on each chloride ion). In the second compound, the iron ion has a 3+ charge, as indicated by the three Cl⁻ ions in the formula. These are two different compounds that need two different names. By the Stock system, the names are iron(II) chloride and iron(III) chloride (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\) Naming the \(FeCl_2\) and \(FeCl_3\) compounds in the modern/stock system.

Name of cation (metal) + (Roman Numeral in parenthesis) + Base name of anion (nonmetal) and -ide

If we were to use the stems and suffixes of the common system, the names would be ferrous chloride and ferric chloride, respectively (Figure \(\PageIndex{3}\)) .

Figure \(\PageIndex{3}\) Naming the \(FeCl_2\) and \(FeCl_3\) compounds in the old/common System.

"Old" base name of cation (metal) and -ic or -ous + Base name of anion (nonmetal) and -ide
-ous (for ions with lower charge)	-ic (for ions with higher charge)

Example \(\PageIndex{4}\):

Name each ionic compound.

1. Co₂O₃
2. FeCl2

Solution

Example \(\PageIndex{4}\): Explanation of the names of ionic compounds

Ionic Compound	Explanation	Answer
Co2O3	We know that cobalt can have more than one possible charge; we just need to determine what it is. Oxide always has a 2− charge, so with three oxide ions, we have a total negative charge of 6−. This means that the two cobalt ions have to contribute 6+, which for two cobalt ions means that each one is 3+. Therefore, the proper name for this ionic compound is cobalt(III) oxide.	cobalt(III) oxide
FeCl2	Iron can also have more than one possible charge. Chloride always has a 1− charge, so with two chloride ions, we have a total negative charge of 2−. This means that the one iron ion must have a 2+charge. Therefore, the proper name for this ionic compound is iron(II) chloride.	iron(II) chloride

Exercise \(\PageIndex{4}\)

Name each ionic compound.

1. AuCl₃
2. PbO₂
3. CuO

Answer a:

gold(III) chloride

Answer b:

lead(IV) oxide

Answer c:

copper(II) oxide

Figure \(\PageIndex{4}\) is a synopsis of how to name simple ionic compounds.

Figure \(\PageIndex{4}\): A Guide to Naming Simple Ionic Compounds.

Exercise \(\PageIndex{5}\)

Name each ionic compound.

1. ZnBr₂
2. Al₂O₃
3. AuF₃
4. AgF

Answer a:

zinc bromide

Answer b:

aluminum oxide

Answer c:

gold(III) fluoride or auric fluoride

Answer d:

silver fluoride

Writing Formulas of Ionic Compounds

Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like \(\ce{NaCl}\), is an empirical formula. This formula merely indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula \(\ce{Na_2S}\). This formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will teach you how to find the correct ratio of ions, so that you can write a correct formula.

If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charge to cancel each other out.

Example \(\PageIndex{5}\): Aluminum Nitride and Lithium Oxide

Write the formula for aluminum nitride and lithium oxide.

Solution

Example \(\PageIndex{5}\): Steps for Problem Solving Writing Formulas

Steps for Problem Solving	Write the formula for aluminum nitride	Write the formula for lithium oxide
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.	\(\ce{Al^{3+}} \: \: \: \: \: \ce{N^{3-}}\)	\(\ce{Li^+} \: \: \: \: \: \ce{O^{2-}}\)
2. Use a multiplier to make the total charge of the cations and anions equal to each other.	total charge of cations = total charge of anions 1(3+) = 1(3-) +3 = -3	total charge of cations = total charge of anions 2(1+) = 1(2-) +2 = -2
3. Use the multipliers as subscript for each ion.	\(\ce{Al_1N_1}\)	\(\ce{Li_2O_1}\)
4. Write the final formula. Leave out all charges and all subscripts that are 1.	\(\ce{AlN}\)	\(\ce{Li_2O}\)

An alternative way to writing a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped.

Example \(\PageIndex{6}\): The Crisscross Method for lead (IV) oxide

Write the formula for lead (IV) oxide.

Solution

Example \(\PageIndex{6}\): Steps For Problem Solving the Crisscross Method for writing formulas.

Crisscross Method	Example: Write the formula for lead (IV) oxide
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.	\(\ce{Pb^{4+}} \: \: \: \: \: \ce{O^{2-}}\)
2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation.
3. Reduce to the lowest ratio.	\(\ce{Pb_2O_4}\)
4. Write the final formula. Leave out all subscripts that are 1.	\(\ce{PbO_2}\)

Exercise \(\PageIndex{6}\)

Write the chemical formula for an ionic compound composed of each pair of ions.

1. the calcium ion and the oxygen ion
2. the 2+ copper ion and the sulfur ion
3. the 1+ copper ion and the sulfur ion

Answer a:

CaO

Answer b:

CuS

Answer c:

Cu₂S

Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions.

Example \(\PageIndex{7}\): Sulfur Compound

Write the formula for sodium combined with sulfur.

Solution

Example \(\PageIndex{7}\): Steps For Problem Solving the Crisscross Method for writing formulas.

Crisscross Method	Write the formula for sodium combined with sulfur
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.	\(\ce{Na^{+}} \: \: \: \: \: \ce{S^{2-}}\)
2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation.
3. Reduce to the lowest ratio.	This step is not necessary.
4. Write the final formula. Leave out all all subscripts that are 1.	\(\ce{Na_2S}\)

Exercise \(\PageIndex{7}\)

Write the formula for each ionic compound.

1. sodium bromide
2. lithium chloride
3. magnesium oxide

Answer a:

NaBr

Answer b:

LiCl

Answer c:

MgO

Recognizing Ionic Compounds

There are two ways to recognize ionic compounds. First, compounds between metal and nonmetal elements are usually ionic. For example, CaBr₂ contains a metallic element (calcium, a group 2 (or 2A) metal) and a nonmetallic element (bromine, a group 17 (or 7A) nonmetal). Therefore, it is most likely an ionic compound. (In fact, it is ionic.) In contrast, the compound NO₂ contains two elements that are both nonmetals (nitrogen, from group 15 (or 5A), and oxygen, from group 16 (or 6A). It is not an ionic compound; it belongs to the category of covalent compounds discussed elsewhere. Also note that this combination of nitrogen and oxygen has no electric charge specified, so it is not the nitrite ion.

Second, if you recognize the formula of a polyatomic ion in a compound, the compound is ionic. For example, if you see the formula Ba(NO₃)₂, you may recognize the “NO₃” part as the nitrate ion, NO₃⁻. (Remember that the convention for writing formulas for ionic compounds is not to include the ionic charge.) This is a clue that the other part of the formula, Ba, is actually the Ba²⁺ ion, with the 2+ charge balancing the overall 2− charge from the two nitrate ions. Thus, this compound is also ionic.

Example \(\PageIndex{8}\)

Identify each compound as ionic or not ionic.

1. Na₂O
2. PCl₃
3. OF₂

Solution

Example \(\PageIndex{8}\): Steps For Problem Solving Identifying Ionic Compounds

Explanation	Answer
a. Sodium is a metal, and oxygen is a nonmetal. Therefore, Na2O is expected to be ionic.	\(Na_2O\), ionic
b. Both phosphorus and chlorine are nonmetals. Therefore, PCl3 is not ionic.	\(PCl_3\), not ionic
c. Both oxygen and fluorine are nonmetals. Therefore, OF2 is not ionic.	\(OF_2\), ionic

Exercise \(\PageIndex{8}\)

Identify each compound as ionic or not ionic.

1. N₂O
2. FeCl₃

Answer a:

not ionic

Answer b:

ionic

Summary

• Ionic compounds are named by stating the cation first, followed by the anion.
• Positive and negative charges must balance.
• Some anions have multiple forms and are named accordingly with the use of roman numerals in parenthesis.
• Formulas for ionic compounds contain the symbols and number of each atom present in a compound in the lowest whole number ratio.

Contributors and Attributions

• Marisa Alviar-Agnew (Sacramento City College)

• Henry Agnew (UC Davis)