This easy to use exercise displays the key functions of the Oxidation Numbers Tutor. Once this exercise is completed, students will be familiar and comfortable with the Tutor, giving them maximum benefit as they progress with their chemistry studies.

The best opportunity to learn occurs when mistakes are made — and that is when the full power of the Quantum Tutor is realized. To demonstrate the Tutor's capabilities and features, some of the more common errors made by students are a key focus of this exercise.

Similar to a human tutor, the Quantum Tutor deliberately offers a variety of wordings and numbers in its examples and explanations every time you use it. The ideas and concepts are the same. So, be prepared — you may not see the exact same words or numbers that appear in these sample screens. Know that you are learning the concepts and the why behind the answers, not just memorizing facts and procedures.

Teachers may choose to print out this document and use it as an assignment in the classroom or for homework. Space has been provided for students to write in their responses.

Learning Objectives:

  • Students will be able to assign oxidation numbers using the Tutor, either by choosing a formula from the menu or by entering their own original work.
  • Students will learn to use the Tutor’s hints and comments in order to learn more about assigning oxidation numbers.

Overview of Exercise:

Steps 1 through 5: Students will enter a formula of their own choosing, ask the Tutor to solve the problem step by step, and receive individualized tutoring along the way.

Step 6: Students will select a formula from the Tutor’s menu and, with the help of the Tutor, assign oxidation numbers to each element.

Step 1: Open the Tutor

Open your favorite web browser, such as Internet Explorer or Netscape. Enter the web address for the Oxidation Numbers Tutor (provided by your teacher) in the location window. At any time you may click Instructions at the upper left of the screen for information about the operation of the Tutor. The main window will look like this:


Figure 1: Oxidation Numbers Tutor main window.


There are two ways for the Tutor to help you assign oxidation numbers. You can either enter your own formula into the top box labeled Enter a Formula, or you can pick a formula from the Choose a Formula menu below it. Either way, the Tutor will provide you with the same detailed hints and comments. Start by entering a sample formula (see Step 2).

Step 2: Enter a formula

The Tutor allows us to enter the formula unformatted, or as plain text. In the box labeled Enter a Formula, enter the following formula as shown:



Figure 2: Entering a formula.


The formula you have just entered is the formula for a common compound available at many supermarkets and most drugstores. The common name for this chemical is Epsom salts. Notice that you don’t need to type in subscripted numbers because the Tutor will properly display them for you.

Step 3: Take the first step

When you have typed the formula, click on the green OK button to begin. You will see a window that looks like this:


Figure 3: The Tutor is ready to begin work on your formula.


In this first problem, we will ask the Tutor to assign the oxidation numbers step by step and explain how it’s done.

Each time you click the green Next Step button, the Tutor will take another step towards a complete solution. Now, take the first step by clicking on the Next Step button. The result will look like this:


Figure 4: The Tutor takes the first step in assigning oxidation numbers.


All the work done by the Tutor will follow a set of oxidation number rules as provided in Table 1 below. These rules can be accessed at any time by clicking the Instructions button located at the upper left of the page.

Table 1: Oxidation Number Rules

First Priority


Free Element Rule

The oxidation number of an atom of a free element equals zero.

Simple Ion Rule

The oxidation number of a monatomic ion equals the charge on the ion.

Fluorine Rule

The oxidation number of fluorine in compounds equals -1.

Hydrogen Rule

The oxidation number of hydrogen in combination with nonmetals equals +1.

Group 1 Metal Rule

The oxidation number of Group 1 metals in compounds equals +1.

Group 2 Metal Rule

The oxidation number of Group 2 metals in compounds equals +2.

Sum Rule

The algebraic sum of the oxidation numbers of all the atoms in a chemical formula equals the net charge on the species.

Separate Ions Rule

In ionic compounds, the oxidation numbers in the cation and the anion are independent and can be assigned separately.

Second Priority


Oxygen Rule

The oxidation number of oxygen in compounds equals -2.

Nonmetal Rule

In binary combinations of nonmetals, the more electronegative element is given a negative oxidation number, equal to the charge on its common monatomic ion.


Notice these rules are divided into two groups, first priority and second priority. It is very important to remember that a second priority rule is only used when there are no applicable first priority rules.

Whenever the Tutor takes a step it always provides an explanation. Carefully following these explanations is the best possible way to learn the rules and become familiar with the logic and concepts involved in assigning oxidation numbers.

Is the order of assigning elements optional? Was there another element that could have been assigned first?

Step 4: Ask for a hint

The Tutor will provide you with useful hints in addition to explanations of its work. These hints relate to the application of the rules and the nature of ionic compounds. Click the green Hint button and you will see a window that looks like this:


Figure 5: The Tutor provides a hint.


The particular hint you get will vary, but all hints remind you of something you can use to determine the next best step. Not only will this help you determine the next step, but it will also help you to realize that assigning oxidation numbers is never random or haphazard. Instead, the procedure is a logical and reasonable process based upon a particular set of rules. It is very important for you to realize that the concepts and procedures you master here will be of enormous benefit to you as you continue your study of chemistry.

Click the green Hint button again and you will see a more detailed hint. However, if you select Hint a third time the computer will take the next step for you and encourage you to try to proceed with the rest of the problem on your own. To see how this works, click Hint a third time and allow the Tutor to take the next step.


Figure 6: The Tutor takes the next step.


Step 5: Splitting an ionic compound

The step the Tutor has taken is to split the cation and anion so they can be solved independently. This is a necessary step but it is easy to overlook since it is frequently done as a mental step. Since the Tutor will not allow you to skip steps, it will insist that you complete this step, especially when a polyatomic ion is involved. It is not difficult to do because it involves exactly the same information used to name and write the formula for the compound. To name this compound using the rules for nomenclature of ionic compounds you must recognize that it is composed of the magnesium ion and the sulfate ion. To write the formula, you must recognize the magnesium ion carries a charge of +2 and the sulfate ion has a charge of -2. You simply instruct the Tutor to separate these two ions by entering:

Split Mg(+2) SO4(-2)

These two ions are now of equal importance and, since magnesium was previously assigned, the Tutor proceeds to assign the elements in the sulfate ion.

When you are working with an ion rather than a neutral compound, what special factors must you consider?

In the sulfate ion, which element do you think the Tutor will assign first?

Continue to click Next Step until the problem is finished.


Figure 7: The Tutor confirms the correct answer.


Now that you are finished, scroll down to the bottom of the screen. There, you will see a record of every step taken and every hint given by the Tutor. This record is a “transcript” of the work you have done so far.


Figure 8: The Tutor always displays the transcript.


It is always there if you need to review when you use the Tutor. Because of the valuable information contained in this transcript, you will find that reviewing it will be very worthwhile. In addition to the correct answers and how to arrive at them, the Tutor also offers excellent suggestions on useful ways to think about the concepts.

Step 6: Start a New Problem

From the lower left of this screen, click on Please enter another formula. This will bring you back to the Oxidation Numbers Tutor main window. You could also click on the New Problem button, located in the upper left corner of the page.

This time, select a formula from the Tutor’s Choose a Formula menu. To do this, click (highlight) the selected formula and then click the green OK button to the immediate right. Choose any formula you wish, but one of the more complex examples near the bottom of the list will make a more interesting test of the Tutor’s capabilities. The Tutor makes no distinction between the formula you entered before and this formula selected from the menu. The Tutor will consistently analyze and help you to solve both using the same techniques.


Figure 9: The Choose a Formula option.


Rather than asking the Tutor to take a step for you, enter your own best answers by typing your entries into the Enter a step box and clicking the OK button.

Feel free to experiment with the features of the Tutor so you will feel completely comfortable with it. Try any series of entries including some answers you know are incorrect, some you are uncertain about, and some you feel are correct and examine the Tutor’s response in each case. Be sure to try some answers you know are unreasonable, such as assigning a value of +5 to a metal or -6 to a nonmetal.

Entering intentional errors like this is one of the very best ways to gain knowledge of the Tutor and the concepts governing the assignment of oxidation numbers. The Tutor will always respond to your work, regardless of the accuracy, and guide you toward productive ways of thinking about the problem. Also, feel free to ask for a Hint at any point in the problem.

What element did the Tutor assign first?

Was there a specific first priority rule applicable to this element?

How does the Tutor respond when you intentionally assign the wrong element first?

What element was assigned last?

Was there any rule, either first or second priority, applicable to this element?

Was it necessary to “split” the ions in order to assign oxidation numbers to this formula?

Explain why the “split” step was or was not necessary in this problem.

How did the Tutor respond when you intentionally entered an unreasonable answer?

Was this response something you should have been aware of already if you had considered it carefully?

The two problems you have just completed illustrate many of the major features of the Quantum Tutors. The program has tutored you through the entire process by encouraging you to bring together related background information and concepts to combine with the rules. In addition to learning the rules for assigning oxidation numbers, you have reviewed ionic charges, chemical symbols, polyatomic ions, formula writing, and how both ionic charges and oxidation numbers are related to the periodic table.

Your challenge is to continue to bring this knowledge together for each new example. With these skills and concepts now clearly associated in your thinking you are much better equipped to handle more and more complex formulas. As you move on to other problems, begin to predict what element should be assigned next and what rule or concept the Tutor will cite as an explanation. When you reach the point where your predictions and explanations begin to match those of the Tutor, then you have mastered this concept and you are ready to apply oxidation numbers to a variety of interesting topics you will encounter as you continue your study of chemistry.