Combining Power The number of electrons that an element can lose, gain, or share is called its Oxidation Number. All the positive ions have to equal the negative ions. For example: NaCl or AlCl 3 In a compound, the sum of the oxidation numbers is ZERO. *
Writing Formulas for Ionic Compounds 1. Write the symbols (write the positive ion first). 2. Write the oxidation number over the top of each symbol.. Reduce if possible. 3. Criss-Cross the oxidation numbers and make them subscripts.