Naming Ionic Compounds

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The chemicals that you will encounter during your chemistry course(s) will undoubtedly fall into many different categories: ionic, binary molecular, acid, coordination compounds, and organic compounds. Each of these classes of compounds uses a unique naming system. In Chem I and II, you will be responsible for naming and writing formulas for commmon ionic compounds, binary molecular compounds, acids, and simple organic compounds.

Since each of these classes of compounds uses a unique naming system, the first step in correctly naming a compound is to identify the type of compound that it is. You can do this by looking at the formula for the compound.

Ionic Compounds: As a rule of thumb, ionic compounds generally consist of a metal and one or more non-metals. The main exception to this are those ionic compounds that contain the ammonium ion (NH₄⁺). All compounds whose formulas begin with NH₄⁺ are also ionic.

Binary Molecular Compounds: As a general rule, binary molecular compounds contain two different non-metals.

Acids: The formulas for inorganic acids (those that do not contain a C-C skeleton) will start with "H" (or H₂ or H₃, etc).

To name an ionic compound you simply need to find the names of the cation and anion present in the compound.

1. Identify the cation:

• The cation is always written first in the formula for an ionic compound.

• Most cations are monoatomic (consist of only 1 atom that has lost electrons to become positively charged) metal ions. In these compounds, the cation will be the first element written.

  • Many (but not all) transition metals plus tin, bismuth, and lead can form more than one cation. Use Roman numerals in parentheses after the name of the element to indicate the charge of such ions.

• The only common polyatomic ion that you will encounter is the ammonium ion (NH₄⁺).

2. Identify the anion:

• Cover up the cation. Everything that is leftover will be the anion.

3. Write the name of the ionic compound by writing the name of the cation followed by the name of the anion.

   • Use the list of ion names from your book, the syllabus, or the hand-out on my web-site to find the names of the ions. Remember, Chem I students are responsible for knowing the names and formulas of the ions listed in the syllabus. Chem II students are responsible for the names of all ions.

Example: Write the correct name for the compound whose empirical formula is Na₂SO₄.

1. Na₂SO₄ is an ionic compound because it contains a metal (Na) and two non-metals (S and O).
2. Identify the cation:
• The cation will be the first element written: Na₂SO₄
  • sodium ion
3. Identify the anion:
• The anion will be everything leftover once the cation has been identified: Na₂SO₄
  • sulfate ion
4. The correct name for Na₂SO₄ is sodium sulfate. Notice that the word "ion" is omitted from the name of the cation and anion when incorporating them into the name of the compound.

Example: Write the correct name for the compound whose empirical formula is (NH₄)₂CO₃.

1. Even though this compound is composed of only non-metals, it is still an ionic compound. (It starts with NH₄.)
2. Identify the cation:
• The cation will always be the first ion written.
  • This compound contains the only common polyatomic cation. (NH₄)₂CO₃
    • ammonium ion
3. Identify the anion:
• The anion will be everything leftover once the cation has been identified: (NH₄)₂CO₃
  • carbonate ion
4. The correct name for this compound is ammonium carbonate.

Example: Write the correct name for the compound whose empirical formula is SnO₂.

1. This compound is an ionic compound. It contains a metal (Sn) and a non-metal (O).
2. Identify the cation:
• The cation is always written first in the formula.
  • SnO₂
• Tin can form two different cations Sn²⁺ and Sn⁴⁺). You must correctly identify its charge.
  • The charges of the cation and the anions must exactly balance out.
  • Since there are two anions and each one has a 2- charge, there is a total negative charge of 4-. In order to balance out the negative charge, the single tin ion must have a 4+ charge.
    • tin (IV) ion
3. Identify the anion:
• The anion will be everything leftover once the cation has been identified.
• SnO₂
  • oxide ion
4. The correct name for the compound is tin (IV) oxide.

Practice Problems
Name the following compounds: MgCl2 KOH Ba(NO3)2 NaHCO3 Pb(SO4)2 CuO NaClO
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