Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 5

Big Idea #3: Chemical Reactions, Energy Changes, and Redox Reactions


The oxidation state (or oxidation number) of an atom indicates the number of electrons that it gains or loses when it forms a bond. For instance, upon forming a bond with another atom, oxygen generally gains two electrons, which are negatively charged, so the oxidation state of oxygen in a bond is −2. Similarly, sodium generally loses one electron when it bonds to another atom, so its oxidation state in a bond is +1. Here are three important things you have to keep in mind when dealing with oxidation numbers.

·        The oxidation state of an atom that is not bonded to an atom of another element is zero. That means either an atom that is not bonded to any other atom or an atom that is bonded to another atom of the same element (like the oxygen atoms in O2).

·        The oxidation numbers for all the atoms in a molecule must add up to zero.

·        The oxidation numbers for all the atoms in a polyatomic ion must add up to the charge on the ion.

Most elements have different oxidation numbers that can vary depending on the molecule that they are a part of. The following chart shows some elements that consistently take on the same oxidation numbers.


Oxidation Number



Alkali metals (Li, Na, …)


Alkaline earths (Be, Mg, …)


Group 3A (B, Al, …)




Halogens (F, Cl, …)


Transition metals can have several oxidation states, which are differentiated from one another by a Roman numeral in the name of the compound. For example, in copper (II) sulfate (CuSO4), the oxidation state for copper is +2, and in lead (II) oxide (PbO), the oxidation state for lead is +2. However, copper can also have an oxidation number of +1, and Pb can range from −4 to +4. In general, transition metals are characterized by variable oxidation states, as are the group 14 metals, tin and lead. Two exceptions to this rule are silver, which always takes on an oxidation state of +1, and zinc, which always takes on an oxidation state of +2.

You should be familiar with the following polyatomic ions and their charges.