Calculate Oxidation Numbers - Online Tutor, Practice Problems & Exam Prep

Oxidation numbers indicate an element's ability to gain, lose, or share electrons, with an oxidation state of 0 in its natural state. Ions have oxidation numbers equal to their charges, while specific rules apply for compounds: Group 1A elements are +1, Group 2A are +2, and oxygen is typically -2 unless in peroxides or superoxides. To find oxidation numbers for non-listed elements, treat them as x, apply known oxidation states, distribute subscripts, and solve for x. Understanding these concepts is crucial for mastering redox reactions and chemical bonding.

Calculate Oxidation Numbers

Now the oxidation number is an important idea as we make our way towards oxidation and reduction reactions, otherwise known as redox reactions. First, we're going to say that the oxidation number itself is an element's ability to gain, lose, or share electrons when alone or in a compound. And we're going to say when it comes to oxidation numbers, it's important to talk about the natural state of an element. For an atom in its natural state, also called its standard state, its oxidation number or oxidation state is equal to 0.

Remember, we have our periodic table here, we know that with our periodic table, there are charges that are unique to different groups, and we know that certain elements exist in certain forms in nature. When it comes to group 1A, we know that the charge is plus 1. Group 2A is plus 2. Group 3A is plus 3. In group 4A, we tend to skip because some elements have varying positive charges, so we're just going to skip that. We know that group 5A is minus 3, then minus 2, then minus 1. And remember, everything is trying to become a noble gas. They're perfect, so they tend not to have charges. If you don't remember this or you haven't seen my videos on it, and you want to explore this a little further, make sure you take a look at my videos on periodic table charges.

Also, remember we've talked about natural states of elements in the past as well. When it comes to elements of the periodic table, we have our diatomic molecules. Those are hydrogen, which is H₂, nitrogen, oxygen, fluorine, chlorine, bromine, and of course, iodine. Also remember that phosphorus tends to exist as P₄ in its natural state when found in nature, and sulfur is S₈, and since selenium is right below sulfur, it is Se₈. These are the natural forms of these different elements when they're found in nature. And remember, all the other elements, I'm not writing them in because in nature, they exist as monoatomic atoms. You can also refresh your memory by taking a look at my periodic table videos when we talk about the natural states of elements. But right now, just realize that if you find an atom in its elemental state, natural state, or standard state, whatever term you're most comfortable with, its oxidation number will be equal to 0.

Calculate Oxidation Numbers Example 1

In this example question it says, which of the following compounds would have an oxidation number or oxidation state equal to 0? So remember, when an element or atom is found in its standard or natural state, its oxidation number or oxidation state is 0. So if we take a look here, we have sodium to begin with, Na. Na, remember, exists as a monoatomic element in nature. Its natural form is just Na, so this would not be equal to 0. Next, we have chlorine. Chlorine exists as a diatomic molecule, exists naturally as Cl₂, so this is that. Helium, one of our noble gases, all the noble gases are monoatomic, meaning they exist by themselves in nature. So this is the natural state of helium and therefore its oxidation number or oxidation state would be equal to 0. So this is our answer. But let's look at the last one. The last one is manganese, and they're saying it's 4. Manganese is one of many elements that exist as monoatomic elements in nature. So its natural state is just Mn. So this would not work. So out of all the choices, only option c would have an oxidation number of 0.

Calculate Oxidation Numbers

Now when an element gains a charge, it's no longer in its standard or natural state, so its oxidation number will no longer be equal to 0. Now, ions, recall, an ion is an element or compound with a positive or negative charge. Remember, your positive ions are called cations and your negative ions are called anions. Now for a monoatomic ion, the oxidation number is equal to its charge. So if they gave us, for example, the aluminum ion, aluminum is in group 3A so its charge is 3+. If we see a charge present for that element, that is also its oxidation number. Well, remember, it has to be in its ion form for the charge and oxidation number to equal one another. That's not always going to be the case. We're going to see later on, there will be cases where a particular element, its oxidation number can range widely based on what it's connected to. But for right now, if you see an ion, you see its charge, that charge is equal to its oxidation number. So keep that in mind as we click on the next video and attempt an example question.

Calculate Oxidation Numbers Example 2

So here it says, which of the following elements would have the most positive oxidation number based on its ionic form? Now, if you've watched my videos on periodic table charges, you'll remember that the periodic table has some key things we need to remember when it comes to particular elements and their charges. For example, silver. Silver is a transition metal. Most transition metals have multiple charges. Silver on the other hand does not. Remember, silver is always plus 1 for its oxidation number. Scandium. Scandium is also a transition metal. It's in group 3b, remember? So group 3b elements. They tend to have a charge of plus 3. Next we have sodium. It's in group 1 a, so its charge is plus 1. And then sulfur is in group 6 a, so its charge is minus 2. We're looking for the most positive oxidation number, and that would have to mean that option b is the correct choice. It has the biggest positive value of plus 3. Remember, if you haven't gotten a chance to look at my periodic table videos on charges, and you just want to go and take a look at the different types of elements and their charges, I highly suggest that you go back and take a look. Because when it comes to charges of elements, it's going to form a foundation for a lot of later topics we're going to cover in chemistry 1 and chemistry 2. Alright. So just remember, in this question the answer would have to be option b, it has the most positive charge and therefore the most positive oxidation.

Which of the following elements would have the lowest oxidation number?

• A. Indium, In
• B. Strontium, Sr
• C. Argon, Ar
• D. Manganese, Mn

Calculate Oxidation Numbers

Like we said before, oxidation numbers don't always correspond to real charges. Therefore, a list of rules will be necessary. Now, we're going to say when different elements are in a compound, these specific rules will be used to calculate our oxidation numbers. Let's take a look at these specific oxidation number rules. When it comes to group 1a elements, they will be plus 1 when connected to any other element. So that's something similar to charges of groups. And we're going to see some similarities between oxidation numbers and charges, but then we're going to see a huge deviation from that. Group 2a, plus 2 when they're connected to another element. Fluorine, it's minus 1 when it's connected to any other element.

Now this is where things start to change. Hydrogen can be either plus 1 or minus 1. It's plus 1 when it's connected to nonmetals. For example, if hydrogen is connected to chlorine, or hydrogen is connected to oxygen, or it's connected to the nonmetal of nitrogen. In all these instances, since hydrogen is connected to a nonmetal, its oxidation number will be plus 1. Now it's negative one when it's connected to a metal or boron. So for example, NaH or CaH₂ or BH₃. In these cases, hydrogen will be minus 1 for its oxidation number.

Now oxygen is even more varied. We're going to say here that oxygen, when it's not a peroxide or superoxide, its oxidation number is minus 2. When it's in its peroxide form, its oxidation number will be minus 1. What exactly is a peroxide? Well, a peroxide, we're going to say is when you have 2 group 1a elements connected to 2 oxygens. So an example, H₂O₂, 2 hydrogens which are in group 1a, 2 oxygens. This would be hydrogen peroxide, Li₂O₂, lithium peroxide, or K₂O₂, potassium peroxide.

Now if oxygen is a superoxide, its oxidation number is minus a half. What is a superoxide? A superoxide is when you have 1 group 1a element with 2 oxygens. So here we could have potassium superoxide, cesium superoxide, sodium superoxide. Just remember, oxygen can vary, so be on the lookout. Do you have a superoxide or a peroxide? If not, its oxidation number is minus 2.

Finally, when we're talking about group 7a, we're talking about chlorine, bromine, and iodine. They are going to be minus 1, except when they're connected to oxygen. In that case, we won't know what their new oxidation number will be, and we'll have to calculate it. So again, we use these specific oxidation number rules when we're talking about different elements connected together. We're going to have to utilize them to determine the oxidation number of any element given to us within a compound.

Calculate Oxidation Numbers Example 3

Here in this example, it asks which compound has oxygen with the lowest oxidation state? If we take a look at the first one, we have NaO₂. We have one group 1A element with 2 oxygens. Remember, that pattern tells us we have a superoxide. And if you have a superoxide, that means the oxidation number of oxygen is minus a half. For the next one, we have CO₂. Now it's not a superoxide. For it to be a superoxide, we would need 1 group 1A element with the 2 oxygens. Carbon is not in group 1A, it's in group 4A, so it's not a superoxide. Next, it's not a peroxide either. To be a peroxide, you need 2 group 1A elements with 2 oxygens. So it's not a peroxide, it's not a superoxide, therefore its oxidation number is minus 2. For the next one, we have Cs₂O₂, that's 2 cesiums which are in group 1A with 2 oxygens. That fits the definition of a peroxide. So, this is a cesium peroxide, so the oxidation number for oxygen would be minus 1. And then finally we have O₂, this is the standard or natural state of oxygen. Remember an element found in its standard or natural state has an oxidation number of 0. So out of all our choices, the one that has the lowest oxidation number, the one with the most negative value would be B, CO₂. So just remember, if we have specific rules for that given element, utilize them to find the correct answer.

Calculate Oxidation Numbers

Now when asked to determine the oxidation number of a non-listed element within a compound, we're going to follow the next 4 steps. Remember, we've gone over the specific oxidation number rules for certain elements. This applies to elements that are not found within that list. So step 1, you're going to treat the non-listed element as x. You're going to use the list to write the known oxidation number of other elements within that compound. For example, if you see oxygen in that compound and it's not a peroxide or superoxide, we know its oxidation number would therefore be minus 2. Step 3, if an element has a subscript, then remember to distribute it. And then step 4, add up the oxidation numbers, create an equation, and make it equal to the charge of the compound. These are the rules that we're going to employ when looking for non-listed elements within a compound. We'll see how we utilize these 4 steps to get to our oxidation number for that particular element. Now that we've gone over them, click on the next video and let's take a look at an example question.

Calculate Oxidation Numbers Example 4

Here it says, give the oxidation number of the carbon atom in the acetate ion. Alright. So we're going to say we're looking for carbon, so carbon is going to be our x. Hydrogen, when it's with nonmetals, is plus 1. And then oxygen, here in it's not a superoxide or a peroxide, so it's minus 2. So we have 2 carbons, each one is x, plus 3 hydrogens, each one is plus 1, plus 2 oxygens, each one is minus 2. This equation equals the charge of the ion which is minus 1. So all we have to do now is solve for x.

2x+3-4=-1

2x - 1 = -1. So add 1 to both sides here. So here, you know, in a weird turn of events, you can see that x now equals 0. So here the oxidation number of carbon within the structure is 0. There are moments where your oxidation number could be 0, and the element is not in its natural state, such as in this example. So again, the oxidation number of carbon, when we calculated it, comes out to be 0.

Here’s what students ask on this topic:

What is the oxidation number of oxygen in H₂O₂?

In H₂O₂ (hydrogen peroxide), oxygen is in its peroxide form. The oxidation number of oxygen in peroxides is -1. This is different from its usual oxidation number of -2 in most compounds. Therefore, the oxidation number of oxygen in H₂O₂ is -1.

How do you determine the oxidation number of sulfur in H₂SO₄?

To determine the oxidation number of sulfur in H₂SO₄ (sulfuric acid), follow these steps:

1. Assign known oxidation numbers: H is +1 and O is -2.

2. Let the oxidation number of S be x.

3. Write the equation: 2(+1) + x + 4(-2) = 0.

4. Solve for x: 2 + x - 8 = 0, so x = +6.

Therefore, the oxidation number of sulfur in H₂SO₄ is +6.

What are the rules for assigning oxidation numbers to elements in compounds?

The rules for assigning oxidation numbers are:

1. Group 1A elements: +1.

2. Group 2A elements: +2.

3. Fluorine: -1.

4. Hydrogen: +1 with nonmetals, -1 with metals.

5. Oxygen: -2, except in peroxides (-1) and superoxides (-1/2).

6. Group 7A elements: -1, except when bonded to oxygen or other halogens.

These rules help determine the oxidation states of elements in various compounds.

How do you calculate the oxidation number of an element in a compound?

To calculate the oxidation number of an element in a compound:

1. Assign oxidation numbers to known elements using standard rules.

2. Let the unknown oxidation number be x.

3. Write an equation summing the oxidation numbers, equal to the compound's charge.

4. Solve for x.

For example, in Fe₂O₃: 2x + 3(-2) = 0, solving gives x = +3. Thus, Fe has an oxidation number of +3.

What is the oxidation number of chlorine in ClO₃^-?

To find the oxidation number of chlorine in ClO₃^-:

1. Assign known oxidation numbers: O is -2.

2. Let the oxidation number of Cl be x.

3. Write the equation: x + 3(-2) = -1.

4. Solve for x: x - 6 = -1, so x = +5.

Therefore, the oxidation number of chlorine in ClO₃^- is +5.