3.5: Ionic Nomenclature

 Learning Objectives

• To use the rules for naming 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. 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.

To distinguish these different ions, 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.

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.

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 polyatomic ions have their own characteristic names, as discussed earlier.

Example \(\PageIndex{1}\)

Name each ion.

1. Ca²⁺
2. S²⁻
3. SO₃²⁻
4. NH₄⁺
5. Cu⁺

Answer a

the calcium ion

Answer b

the sulfide ion (from Table \(\PageIndex{2}\) )

Answer c

the sulfite ion

Answer d

the ammonium ion

Answer e

the copper(I) ion. Copper can form cations with either a 1+ or 2+ charge, so we have to specify which the charge of the ion

Exercise \(\PageIndex{1}\)

Name each ion.

1. Fe²⁺
2. Fe³⁺
3. SO₄²⁻
4. Ba²⁺
5. HCO₃⁻

Answer a

the iron (II)

Answer b

the iron (III)

Answer c

the sulfate ion

Answer d

the barium ion

Answer e

the bicarbonate ion or hydrogen carbonate ion

Example \(\PageIndex{2}\)

Write the formula for each ion.

1. the bromide ion
2. the phosphate ion
3. the copper(II) ion
4. the magnesium ion

Answer a

Br⁻

Answer b

PO₄³⁻

Answer c

Cu²⁺

Answer d

Mg²⁺

Exercise \(\PageIndex{2}\)

Write the formula for each ion.

1. the fluoride ion
2. the carbonate ion
3. the iron(II) ion
4. the potassium ion

Answer a

F⁻

Answer b

CO₃^2-

Answer c

Fe²⁺

Answer d

K⁺

Naming Compounds

Now that we know how to name ions, we are ready to name ionic compounds. We do so by placing the name of the cation first, followed by the name of the anion, and dropping the word ion from both parts. For example, what is the name of the compound whose formula is \(\ce{Ba(NO3)2}\)?

The compound’s name does not indicate that there are two nitrate ions for every barium ion. You must determine the relative numbers of ions by balancing the positive and negative charges.

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 \(\ce{FeCl2}\) and \(\ce{FeCl3}\). In the first compound, the iron ion has a 2+ charge because there are two \(\ce{Cl^{−}}\) 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 \(\ce{Cl^{−}}\) ions in the formula. 

Example \(\PageIndex{3}\)

Name each ionic compound, using Roman numerals if necessary.

1. Ca₃(PO₄)₂
2. (NH₄)₂Cr₂O₇
3. KCl
4. CuCl
5. SnF₂

Answer a

calcium phosphate

Answer b

ammonium dichromate (the prefix di- is part of the name of the anion)

Answer c

potassium chloride

Answer d

copper(I) chloride

Answer e

tin(II) fluoride

Exercise \(\PageIndex{3}\)

Name each ionic compound, using Roman numerals if necessary.

1. ZnBr₂
2. Fe(NO₃)₃
3. Al₂O₃
4. CuF₂
5. AgF

Answer a

zinc bromide

Answer b

iron (III) nitrate

Answer c

aluminum oxide

Answer d

copper (II) fluoride

Answer e

silver fluoride

Figure \(\PageIndex{1}\) is a synopsis of how to name simple ionic compounds.

Figure \(\PageIndex{1}\): A Guide to Naming Simple Ionic Compounds. Follow these steps to name a simple ionic compound.

Identify the cation name and the anion name. If the cation can have more than one possible charge, use the roman numeral name of the cation and name of the anion. Examples of this would be FeCl₂, which is iron(II) chloride, CuSO₄, which is copper(II) sulfate, and Cr₂O₃, which is chromium(III) oxide. If the cation can not have more than one possible charge, use the name of the cation and the name of the anion. Examples of this would be KBr, which is potassium bromide, NaNO₃, which is sodium nitrate, and (NH₄)₂S, which is ammonium sulfide.

KEY TAKEAWAY

• Each ionic compound has its own unique name that comes from the names of the ions.

EXERCISES

1. Briefly describe the process for naming an ionic compound.
2. In what order do the names of ions appear in the names of ionic compounds?
3. Name each ion.

1. Ra²⁺
2. P³⁻
3. H₂PO₄⁻
4. Sn⁴⁺

4. Name each ion.

1. Cs⁺
2. As³⁻
3. HSO₄⁻
4. Sn²⁺

5. Name the ionic compound formed by each pair of ions.

1. Na⁺ and Br⁻
2. Mg²⁺ and Br⁻
3. Mg²⁺ and S²⁻

6. Name the ionic compound formed by each pair of ions.

1. K⁺ and Cl⁻
2. Mg²⁺ and Cl⁻
3. Mg²⁺ and Se²⁻

7. Name the ionic compound formed by each pair of ions.

1. Na⁺ and N³⁻
2. Mg²⁺ and N³⁻
3. Al³⁺ and S²⁻

8. Name the ionic compound formed by each pair of ions.

1. Li⁺ and N³⁻
2. Mg²⁺ and P³⁻
3. Li⁺ and P³⁻

9. Name the ionic compound formed by each pair of ions. Use both the Stock and common systems, where appropriate.

1. Fe³⁺ and Br⁻
2. Fe²⁺ and Br⁻
3. Au³⁺ and S²⁻
4. Au⁺ and S²⁻

10. Name the ionic compound formed by each pair of ions. Use both the Stock and common systems, where appropriate.

1. Cr³⁺ and O²⁻
2. Cr²⁺ and O²⁻
3. Pb²⁺ and Cl⁻
4. Pb⁴⁺ and Cl⁻

11. Name the ionic compound formed by each pair of ions. Use both the Stock and common systems, where appropriate.

1. Cr³⁺ and NO₃⁻
2. Fe²⁺ and PO₄³⁻
3. Ca²⁺ and CrO₄²⁻
4. Al³⁺ and OH⁻

12. Name the ionic compound formed by each pair of ions. Use both the Stock and common systems, where appropriate.

1. NH₄⁺ and NO₃⁻
2. K⁺ and Cr₂O₇²⁻
3. Cu⁺ and CO₃²⁻
4. Na⁺ and HCO₃⁻

13. Give two names for each compound.

1. Al(HSO₄)₃
2. Mg(HSO₄)₂

14. Give two names for each compound.

1. Co(HCO₃)₂
2. LiHCO₃

Answers

1. Name the cation and then the anion but don’t use numerical prefixes.
2. the cation name followed by the anion name

3.

1. the radium ion
2. the phosphide ion
3. the dihydrogen phosphate ion
4. the tin(IV) ion

4.

1. the cesium ion
2. the arsenide ion
3. the hydrogen sulfate ion
4. the tin(II) ion

5.

1. sodium bromide
2. magnesium bromide
3. magnesium sulfide

6.

1. potassium chloride
2. magnesium chloride
3. magnesium selenide

7.

1. sodium nitride
2. magnesium nitride
3. aluminum sulfide

8.

1. lithium nitride
2. magnesium phosphide
3. lithium phosphide

9.

1. iron(III) bromide
2. iron(II) bromide
3. gold(III) sulfide
4. gold(I) sulfide

10.

1. chromium(III) oxide
2. chromium(II) oxide
3. lead(II) chloride
4. lead(IV) chloride

11.

1. chromium(III) nitrate
2. iron(II) phosphate
3. calcium chromate
4. aluminum hydroxide

12.

1. ammonium nitrate
2. potassium dichromate
3. copper(I) carbonate
4. sodium hydrogen carbonate or sodium bicarbonate

13.

1. aluminum hydrogen sulfate or aluminum bisulfate
2. magnesium hydrogen sulfate or magnesium bisulfate

14.

1. cobalt hydrogen carbonate or cobalt bicarbonate
2. lithium hydrogen carbonate or lithium bicarbonate