Chemical Formulas

The nature of any ionic compound can be accurately described by writing its chemical formula, called the ionic formula unit, which lists the simplest whole number ratio of metal to nonmetal within the lattice structure of the compound. This notation uses a combination of atomic symbols and simple whole numbers, which are written as subscripts, to indicate the simplest whole number ratio of metal ions to nonmetal ions in the ionic compound formed.

Example:

atomic symbols identifying element

X Y₂

subscripts, identifying simple whole number ratio of the atoms in the compound. By convention the subscript,"1", is never written.

The following provides one with simple guidelines, based on the concepts presented previously, which allows one to quickly write correct formulas of ionic compounds formed by the oxidation-reduction reaction involving metals and nonmetals.

(1) for any metal/nonmetal combination, list their symbols along with the proper oxidation state of the ions formed. Examples:

Na⁺¹ O^-2 Al⁺³ F^-1 Ca⁺² O^-2

(2) diagonally "switch" the oxidation numbers to subscripts.

Na⁺¹ O^-2 Al⁺³ F^-1 Ca⁺² O^-2

Na₂O AlF₃ Ca₂O₂

note: by convention, the subscript "1" is not written

(3) and if needed, reduce to the simplest whole number ratio. Example:

This method always predicts the correct ratio of metal and nonmetal ions in an ionic compound. It also provides one with information on the total number of electrons transferred between the atoms