Ch.4 - Chemical Quantities & Aqueous ReactionsWorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds
Jules Bruno

Calculating oxidation number

Redox reactions involve a reactant being oxidized while another reactant is reduced. But in order to understand who’s been oxidized and who’s been reduced, you first must master calculating the oxidation numbers of compounds as well as elements. Without being able to do this, you won’t know for sure who’s been oxidized and who’s been reduced. When we talk about calculating the oxidation number, we break it down into two categories. We have our general rules and we have our specific rules. 

How to calculate oxidation number 

When it comes to general rules, we’re talking about calculating the oxidation number of an entire element or a compound. 

For an atom in its elemental form, the oxidation number will equal to zero. When we say elemental form, that means that the element is either by itself like we have a carbon (solid), or it’s connected to copies of itself like Cl2, P4 and S8. This is also important. Not only does the element need to be either by itself or with copies of itself, but it must not have any charge present. They must be neutral. If that’s true, then its oxidation number will be zero. Each of these have an oxidation number of zero. 

For an ion, the oxidation number equals the charge. Here, oxygen is by itself but it has a charge so its oxidation number now is -2. Sodium here, its oxidation number is equal to its charge so it’d be +1. Here phosphate, the entire polyatomic ion has an oxidation number of -3. Then finally, ammonia, if we’re looking at the entire compound, its oxidation number is equal to 0 because it has no charge present. 

When we talk about specific rules, that’s when we’re talking about finding the individual oxidation numbers of elements within a molecule or compound.

If we have a Group 1A ion that’s part of a compound, its oxidation number is +1. If we have a Group 2A element within a compound, its individual oxidation number is +2. Then things start to get a little bit different from what we’re accustomed to seeing with charges.

Oxidation number of hydrogen 

Here for hydrogen, it is +1 when it’s connected to nonmetals. Here, hydrogen is connected to oxygen so it’s +1, it’s still +1 (NH3), and here it’s +1 (HBr). But once that hydrogen connects to a metal or to boron, its oxidation now becomes -1. In these cases, hydrogen is connected either to a metal or to a boron. 

Oxidation number of fluorine

When we look at Fluorine within a compound, It doesn't care what’s around it. If fluorine is within a compound, its individual oxidation number will always be -1.

Oxidation number of oxygen

With oxygen, it gets a little bit crazier. 


With oxygen, if it’s a peroxide, it’s gonna be -1 for its oxidation number. A peroxide has the formula of X2O2, where X represents a Group 1A element. Here we have hydrogen peroxide, sodium peroxide and potassium peroxide. Notice the setup. It’s two elements from Group 1A connected to two oxygens. That’s what makes a peroxide. 

If it’s a superoxide, its oxidation number will be -½. What’s the general setup of a superoxide? A superoxide is just one element from Group 1A connected to two oxygens. Remember, X represents a Group 1A element. Here we have potassium superoxide, cesium superoxide and lithium superoxide. In these cases, oxygen would be -½.

If oxygen is not in a peroxide or a superoxide, then its oxidation number will equal -2.

Oxidation number of halogens

Finally, halogens, which are elements in Group 7A. They are -1 except when they’re connected to oxygen. 

When they’re connected to oxygen, we don’t know what their new oxidation number will be. We will have to calculate it. The exception to this is go back to fluorine. Fluorine doesn't even care if it’s connected to oxygen. It’s still always gonna be -1 when it’s within a compound. 

Summary

Again, it’s essential that you learn these rules for calculating the oxidation number of compounds as a whole as well as individual elements within a compound. This is the first step to really understanding redox reaction. It’s highly suggested that you memorize these rules. Then when we talk about redox reactions, remember the fundamental features of oxidation versus reduction.


Jules Bruno

Jules felt a void in his life after his English degree from Duke, so he started tutoring in 2007 and got a B.S. in Chemistry from FIU. He’s exceptionally skilled at making concepts dead simple and helping students in covalent bonds of knowledge.