The Rules for Naming Ionic Compounds

Rules for naming ionic compounds:

  1. Always name the metal ion first.

  1. Name the nonmetal ion second.

  1. Change the ending of the nonmetal or second element by adding -ide.

    For example:

    • chlorine is changed to chloride

    • phosphorus is changed to phosphide

  1. Use lower case letters for the compound name.

    For example:

    • The name of the formula CaCl2 is calcium chloride.

    • Magnesium and oxygen will combine to form magnesium oxide.

The Rules for Writing Formulas for Ionic Compounds

When metal and nonmetal ions form ionic bonds, their total electrical charges must balance each other exactly (as in, add up to zero). In order to write a correct formula, you must know the ion charges of all the atoms involved. This is where your table of elements comes in very handy—to find the ion charges of each element.

Rules for writing chemical formulas for ionic compounds:

Following the above rules, this is the way you would write the formula for lithium chloride:

  1. Lithium is written Li. It has a charge of +1.

  2. Chloride is Chlorine, and is written Cl. It has a charge of -1.

Since they both have the same charge already, the formula is LiCl. Do not write “1” beside the symbols.

Following the above rules, this is the way you would write the formula for sodium sulphide:

  1. Sodium is written Na. It has a charge of + 1.

  2. Sulphide is sulphur, and is written S. It has a charge of -2.

The total positive charges must be the same as negative. One sulphur atom has a charge of -2. Since sodium has charge of +1, we will need 2 sodium atoms to make the total charge of sodium equal +2, to balance out the charge. The formula for sodium sulphide is written Na2S.

Read This

Now read BC Science 9 - Names and Formulas of Ionic Compounds, pages 84 to 87 or BC Science Probe 9 - Ion charge and the Formulas of Ionic Compounds, pages 238 to 240. This is very important information, so take the time to do these readings. You need to understand the rules!

Criss-cross Method

There is a short cut for writing formulas for ionic compounds, called the criss-cross method. It is an easy way to get the subscripts right for each ion, although not the most scientific method!

Have a look at this example to see how the formula for calcium phosphide is determined using the criss-cross rule.

Criss-cross method example

The formula for calcium phosphide is Ca3P2.


Copyright 2007, Open School BC