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Binary Ionic Compounds

Rules for naming monatomic metal and nonmetal ions

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Naming Binary Ionic Compounds and Formulas

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Naming Binary Ionic Compounds

A binary ionic compund is one that is composed only of ions of monatomic cations and a monatomic anions.

Tip: In writing or reading ionic formulas, note that the cation is always written first.

You can remember this by remembering that "The CAT has an ANION (an eye on) the formula."

To name a binary ionic compound, name it by the name of the cation followed by the name of the anion. For example, NaCl is sodium chloride. 

Writing Formulas

Beware! Just because an ionic compound is binary does not mean that it only consists of two ions— one anion and one cation. Instead, make sure that the total charges of the ions in the compound cancel. For example, Mg2+ and F- would form MgF2.

Notice that you are trying to equalize the charges of the ions in the compound so the charges as denoted by the ions are omitted in the compound's formula. Indicate how many of each ion is in the formula by using a subscript to the right of the ion, and make sure that the ratio of ions is reduced to the lowest whole number ratio.

Method: You may try to use a criss-cross method as show below to help you orient yourself when writing formulas.

License: CC BY-NC 3.0


How would you name the following? (highlight to the right of each formula for answers):

MgCl2    Magnesium chloride

NaF      Sodium Fluoride

Fe2O3    Iron (III) Oxide or Ferric Oxide

Write the formulas for the following (highlight to the right of each name for answers):

Magnesium Fluoride      MgF

Cobaltous Chloride  or Cobalt (II) Chloride     CoCl2

Aluminum Oxide     Al2O3

Explain why CaCl is not a correct formula.

Explain why Na+Cl- is not a correct formula.

For guidance in answering these questions or for more information on naming binary ionic compounds, look here.

For more information on writing formulas, look here.

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