3.4 Ionic Nomenclature

Learning Objectives

1. Use the rules for naming ionic compounds.

Naming Ions

Naming the Cation

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.  Metals ions from Group 1 always have a 1+ charge, metals ions from Group 2 always have a 2+ charge, the only aluminum ion is 3+, zinc ions are  2+, and silver ions are 1+.  These are the fixed-charge metals.  The names for fixed-charge metal ions must never be given a Roman numeral.

Metals that can have several different charges as ions are referred to as variable-charge metals.  Almost all transition metals plus the metals under the staircase are variable-charge metals. Iron, for example, can form two different cations, each of which, when combined with the same anion, makes a different compound with unique physical and chemical properties.  For example, FeCl2 and FeCl3 differ in density, melting point, solubility, and color.  Thus, we need a different name for each iron ion to distinguish Fe2+ from Fe3+. The same issue arises for all of the variable-charge metals.

 In the modern approach to naming compounds containing variable-charge metals, called the Stock system, a variable-charge metal 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. The names for variable-charge metals must always be given a Roman numeral.

An older system used the endings -ic for when the metal had the larger of its possible negative charges and -ous for when the metal had the smaller of its possible negative charges instead of the Roman numerals.  We will not use this older system.

Naming the Anion

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 3.3 “Some Monatomic Anions” lists the names of some common monatomic ions.

Table 3.3 Some Monatomic Anions
Ion Name
F− fluoride ion
Cl− chloride ion
Br− bromide ion
I− iodide ion
O2− oxide ion
S2− sulfide ion
P3− phosphide ion
N3− nitride ion

The polyatomic ions have their own characteristic names, as we saw in Table 3.1 “Some Polyatomic Ions”.

Example 6

Name each ion.

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

Solution

SKill Building Exercise

Name each Ion

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

Example 7

Write the formula for each ion.

  1. the bromide ion
  2. the phosphate ion
  3. the copper 1+ ion
  4. the magnesium ion

Solution

SKILL-BUILDING EXERCISE

Write the formula for each ion.

  1. the fluoride ion
  2. the carbonate ion
  3. the tin4+ ion
  4. the potassium ion

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 Ba(NO3)2?

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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 FeCl2 and FeCl3. 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.

Example 8

Name each ionic compound.  Use Roman numerals only when needed.

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

Solution

Skill-building Exercise

Name each ionic compound. Use Roman numerals only when neccessary.

  1. ZnBr2
  2. Fe(NO3)3
  3. Al2O3
  4. AuF3
  5. AgF

Figure 3.7 “A Guide to Naming Simple Ionic Compounds” is a synopsis of how to name simple ionic compounds.

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Figure 3.7 A Guide to Naming Simple Ionic Compounds. Follow these steps to name a simple ionic compound.

Concept Review 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. When do you need a Roman numeral in the name of an ionic compound? Give an example.  When are you not allowed to have a Roman numeral in an ion compound name?

 

ANswers

Key Takeaway

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

Exercises

  1. Name each ion.

    1. Ra2+
    2. P3−
    3. H2PO4−
    4. Sn4+

  2. Name each ion.

    1. Cs+
    2. As3−
    3. HSO4−
    4. Sn2+

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

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

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

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

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

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

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

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

  7. 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−

  8. 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−

  9. 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−

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

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

  11. Name each compound.

    1. Al(HSO4)3
    2. Mn(HSO4)2

  12. Name each compound.

    1. Co(HCO3)2
    2. LiHCO3

answers