You probably recall learning about ions in Biology.  An ion is a charged particle formed when a neutral atom or group of atoms gain or lose one or more electrons.  When a single atom forms an ion, as in the case of Al⁺³, it is called a monatomic ion.   When of group of atoms that are covalently bonded together form an ion, as in the case of NH₄⁺, it is called a polyatomic ion.

     Sometimes ions with opposite charges are attracted together and will form ionic compounds.  Table Salt, NaCl is such a compound formed from Na⁺ ions and Cl^- ions.  Neutral atoms can also form compounds when they join together, as in the case of water (H₂O).  However, since these compounds are not composed of ions, they are called molecular compounds.  You will learn more about these types of compounds in lesson 5-3. 

     Regardless as to whether a compound is made up of ions or not, each atom in the compound has an apparent charge.  This apparent charge, called the oxidation number, represents the charge that an atom would have if electrons were transferred completely to the atom with the greater attraction for them in a given situation.  These oxidation numbers can be used to predict the ratio by which atoms will combine when they form compounds.

The following rules help us assign the oxidation number of elements:

     Now, in time you will find it easy to predict many oxidation numbers, as you become more familiar with the periodic table and the rules above.  Until that time, you should make use of reference tables that list the oxidation numbers of common ions.  Depending on your teacher, he or she may allow you to make use of such tables for quizzes and exams.  For your  convenience, I will provide examples of these tables below.  Keep in mind that the table that your teacher uses may differ from the ones provided below.

     Now, some elements show different positive oxidation numbers, in different situations.  The stock system, which you will learn more about in lessons 4-3 and 4-4, uses Roman numerals to show the oxidation number of the element.  For example, Lead(II) is lead with an oxidation number of +2.   Chromium(III) is Chromium with an oxidation number of +3.  The oxidation number for these types of elements will always be positive.  I provide a table below, but once you understand the stock system you will not need the table any longer.