Although ionic compounds are composed of both positively and negatively charged ions, the overall compound and its formula are electrically neutral. In other words:

total positive charge (from cation) = total negative charge (from anion)

Forgetting to follow this rule is one of the most common mistakes that students make when writing formulas for ionic compounds.

The empirical formula for an ionic compound indicates the smallest whole number ratio of cations and anions needed to produce an electrically neutral compound. The empirical formula is written with the cation first followed by the anion.

To write the empirical formula for an ionic compound:

Identify the cation.

Cation is written first in the name of the compound.

Write the correct formula and charge for the cation.

Identify the anion.

Anion is written last in the name of the compound.

Write the correct formula and charge for the anion.

Combine the cation and anion to produce anelectrically neutral compound.

If the charges on the cation and anion are equal in magnitude (i.e. +1/-1, +2/-2, +3/-3), combine the cation and anion in a 1:1 ratio.

If the charges on the cation and anion are NOT equal in magnitude, use the charge on the cation as the subscript for the anion. Use the charge on the anion (omitting the negative sign) as the subscript for the cation.

Place parentheses around a polyatomic ion if you need more than one of them in the final formula.

Do not show the charges of the ions when you write the final formula for the compound.

Make sure that the subscripts for the cation and anion are the smallest whole number ratio.