5.5: 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. When creating ionic compounds there are three specific circumstances with individual naming rules: binary ionic compounds, ionic compounds containing a metal with a variable charge (transition metal), and polyatomic ionic compounds. Each will be described below.

Binary Compounds

Binary ionic compounds involve only two separate elements, a metal and a nonmetal, and follow the following general rules.

1. Identify the metal (cation) and the nonmetal (anion).

2. Name the metal by it's elemental name without any changes. Example: Na ⁺ = Sodium, Mg ²⁺ = Magnesium

3. Name the nonmetal by using the first syllable of it's elemental name and adding the ending "ide". Example: Chlorine (Cl^-) = Chloride

4. Write the name of the metal first and nonmetal second. Example: Sodium Chloride (NaCl) and Magnesium Chloride (MgCl₂)

Metals with Multiple Charges

We have seen that some elements in the transition metals 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⁺. Therefore, when naming ionic compounds containing a metal that may possess variable charges roman numerals will be placed after the metal name reflecting it's charge. Some common examples can be seen below in Table \(\PageIndex{1}\).

Table \(\PageIndex{1}\): Examples of Ionic Compounds Containing Metals of Variable Charges

Ionic Formula	Name of Compound
FeCl2	Iron (II) Chloride
FeCl3	Iron (III) Chloride
Cu2O	Copper (I) Oxide
CuO	Copper (II) Oxide

Determining the charge, and thus the roman numerals, on the metal ion may be difficult initially. However, keep in mind that you know the nonmetal charge and can determine the value of the metal based on the Ionic Formula. For example, the ionic formula FeCl₂ contains 2 chlorine atoms but only 1 iron atom. Since chlorine has a charge of -1 and there are 2 atoms there is an overall -2 charge on the compound. Since there is only 1 iron atom the overall positive charge must be equal to +2 to create a neutral compound and the name is Iron (II) Chloride.

Ionic Compounds Containing Polyatomic Ions

Binary Ionic compounds were compounds containing only two elements that were attracted to each other by electrostatic interactions. However, there is another class of ionic compounds created with multiple elements. Polyatomic ions, also referred to as oxyanions because they often contain oxygen, are created from groups of covalently bound atoms containing an overall charge. Table \(\PageIndex{2}\) lists the names of some common polyatomic ions.

Table \(\PageIndex{2}\): Some Polyatomic Ions. Bolded Ions represent the most common polyatomic ion for that element

Nonmetal	Formula of Ion	Name of Ion
Hydrogen	OH^-	Hydroxide Ion
Carbon	CO3^2^- HCO3^- CN^- CH3COO^-	Carbonate Hydrogen Carbonate (Bicarbonate) Cyanide Acetate
Chlorine	ClO4^- ClO3^- ClO2^- ClO^-	Perchlorate Chlorate Chlorite Hypochlorite
Nitrogen	NH4^+ NO3^- NO2^-	Ammonium Nitrate Nitrite
Sulfur	SO4^2^- HSO4^- SO3^2^- HSO3^-	Sulfate Hydrogen Sulfate (or Bisulfate) Sulfite Hydrogen Sulfite (or Bisulfite)
Phosphorous	PO4^3^- HPO4^2^- H2PO4^- PO3^3^-	Phosphate Hydrogen Phosphate Dihydrogen Phosphate Phosphite

Note that the most common polyatomic ion form for each element contains the ending "ate". If one oxygen is removed, without changing the charge of the overall compound, the ending is changed to the "ite" ending. Example: Phosphate = PO₄^3- and Phosphite = PO₃^3-. These endings represent the number of oxygen contained in the compound and therefore cannot be removed or dropped from the name when they're incorporated into an ionic compound. Naming ionic compounds will follow these rules.

Naming polyatomic ions:

Identify the cation and the anion of the compound. See rules below if the cation is ammonium.

a. If the cation is a metal without variable charge, name as above with its elemental name.

Examples: NaNO₃ = Sodium Nitrate, CaCO₃ = Calcium Carbonate

b. If the cation is a metal with variable charge, name as above with the associate roman numeral showing its charge.

Examples: CuCO₃ = Copper (II) Carbonate, Fe(OH)₃ = Iron (III) Hydroxide

Naming compounds with Ammonium:

Identify the anion associated with the ammonium ion.

a. If the nonmetal is a single element (Ex. N, O, P, or Halogen) then name as before with the first syllable of the element followed by the "ide" ending.

Examples: NH₄Cl = Ammonium Chloride, (NH₄)₂O = Ammonium Oxide.

Note that when writing the formula's of compounds containing polyatomic ions that parenthesis are needed when more than one ion is involved in the compound.

b. If ammonium is present with a second polyatomic ion, name the compound with no changes to either polyatomic ion name.

Examples: NH₄NO₃ = Ammonium Nitrate, (NH₄)₂CO₃ = Ammonium Carbonate

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

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 charge this ion has)

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 potassium ion

Answer a

F⁻

Answer b

CO₃^2-

Answer d

K⁺

Example \(\PageIndex{3}\)

Name each ionic compound.

1. Ca₃(PO₄)₂
2. KCl
3. CuCl
4. SnF₂

Answer a

calcium phosphate

Answer c

potassium chloride

Answer d

copper(I) chloride

Answer e

tin(II) fluoride

Exercise \(\PageIndex{3}\)

Name each ionic compound, using both Stock and common systems 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, either use the Stock system name of the cation and name of the anion, or use the stem of the cation name and -ic/-ous and the name of the anion. Examples of this would be FeCl₂, which is iron(II) chloride of ferrous chloride, CuSO₄, which is copper(II) sulfate or cupric sulfate, and Cr₂O₃, which is chromium(III) oxide or chromic 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.

• In what order do the names of ions appear in the names of ionic compounds?

3. Which ionic compounds can be named using two different systems? Give an example.

4. Name each ion.

1. Ra²⁺
2. P³⁻
3. H₂PO₄⁻
4. Sn⁴⁺

5. Name each ion.

1. Cs⁺
2. As³⁻
3. HSO₄⁻
4. Sn²⁺

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

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

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

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

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

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

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

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

10. 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⁻

11. 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⁻

12. 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⁻

13. 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⁻

14. Give two names for each compound.

1. Al(HSO₄)₃
2. Mg(HSO₄)₂

15. 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. Ionic compounds in which the cation can have more than one possible charge have two naming systems. FeCl₃ is iron(III) chloride.

4.

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

5.

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

6.

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

7.

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

8.

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

9.

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

10.

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

11.

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

12.

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

13.

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

14.

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

15.

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