4.8 • +11k Ratings
Save
Print
Edit
Electrons can be lost or gained when some atoms interact with other atoms and bond or react with them. Why are oxidation numbers important in this context?
Oxidation numbers are used by chemists to deduce and keep track of the number of electrons transferred or shared during chemical reactions. Oxidation numbers are also useful to chemists when it comes to naming inorganic compounds.
Firstly, we will define the term oxidation.
Then, we will look at the oxidation number rules.
After that, we will explore how oxidation numbers relate to naming compounds.
Finally, we shall work out the oxidation numbers of elements in compounds and ions.
Remember that oxidation is a process in which electrons are lost. Therefore, oxidation numbers or oxidation state, is the total number of electrons that a compound exchanges (gains or loses) in order to form a chemical bond.
There are a few rules that can help and simplify the way we work out oxidation numbers.
For more information on trends, check out Periodic Trends. Note that when talking about oxidation numbers the sign of the number goes before it, while the ionic charge goes as a superindex with the sign of the charge after the number.
Lots of elements have the same oxidation number in all of their compounds.
To help with working out the oxidation numbers of different compounds, here is an image of the periodic table with the oxidation numbers per group.
A periodic table with the oxidation numbers of the elements within their groups. Sahraan Khowaja, StudySmarter
As we've learned, there are a few exceptions to the oxidation numbers of elements within compounds.
In metal hydrides, hydrogen has an oxidation number of -1. An example of a metal hydride is sodium hydride (NaH) or potassium hydride (KH). Remember how we said the oxidation number of a neutral compound is zero and that group 1 metals are always +1? This means that in a metal hydride hydrogen must be -1, as the metal is +1.
Oxygen has the oxidation number -1 in peroxides. A key example of a peroxide is hydrogen peroxide, H2O2. Once again, this is a neutral compound, therefore the sum of the oxidation numbers must be zero. In the case of H2O2, each hydrogen atom has the oxidation number +1, so each oxygen atom must have the oxidation number -1.
In F2O, the more electronegative element is fluorine, so it gains the negative oxidation number -1. There are two atoms of fluorine, so the oxidation number is doubled to -2. Therefore, the oxidation number of oxygen is +2.
Some elements have more than one possible oxidation number. If this is the case, the specific oxidation number in a given compound is specified using roman numerals. It is also indicated in the names of the compounds. For example, (I) = +1, (II) = +2 and (III) = +3.
Let’s have a look at some compounds to elaborate on this. One example is iron (II) sulfate, FeSO4. In this compound, iron has the oxidation number +2, so the compound contains Fe2+ ions. Similarly, another example is iron (III) sulfate, Fe2(SO4)3. In this compound, iron has the oxidation number +3, so the compound contains Fe3+ ions.
Commonly, the ending of ions containing oxygen and another element is '- ate' and '- ite'. Examples of these are sulfate, SO42-, and sulfite, SO32-; and nitrate, NO3-, and nitrite, NO2-.
Acids that contain oxygen, also known as ‘oxoacids’, end in '- ic' or '- ous'. Examples of these are sulfuric acid, H2SO4, and sulfurous acid, H2SO3.
Your turn: have a go at working out oxidation numbers of some elements using the rules we have covered. The solutions will follow the questions below.
What are the oxidation numbers of sulfur in the following compounds or ions:
1. S8
Because this is an uncombined element, the oxidation number is 0.
2. H2S
In this compound, hydrogen has the oxidation number +2 as there are two atoms of hydrogen (2 x+1). Therefore, sulfur must have the oxidation number -2.
3. SO32-
There are three oxygen atoms here, so the oxidation number of oxygen will be (-2 x 3) = -6. The overall charge on the compound is -2. Therefore, the oxidation number of sulfur must be +4.
4. H2SO4
In this compound, there are four oxygen atoms, so -2 x 4 = -8. There are two hydrogen atoms, so +1 x 2 = +2. The overall charge on the compound is zero. So, the oxidation number of sulfur must be +6.
A number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element in the compound.
Oxidation numbers show the total number of electrons that have been removed from an element or added to an element to get to its present state.
In an ion or a compound, the element that is more electronegative is given the negative oxidation number. The less electronegative element is given the positive oxidation number.
- The oxidation number of all uncombined elements is zero.
- The oxidation number of monatomic ions is the same as the ionic charge.
- The oxidation number of a neutral compound is zero.
- The sum of the oxidation numbers in an ion is equal to the overall charge of the ion.
- In an ion or a compound, the element that is more electronegative is given the negative oxidation number.
In Cl2 (chlorine gas) the oxidation number of chlorine is 0.
Be perfectly prepared on time with an individual plan.
Test your knowledge with gamified quizzes.
Create and find flashcards in record time.
Create beautiful notes faster than ever before.
Have all your study materials in one place.
Upload unlimited documents and save them online.
Identify your study strength and weaknesses.
Set individual study goals and earn points reaching them.
Stop procrastinating with our study reminders.
Earn points, unlock badges and level up while studying.
Create flashcards in notes completely automatically.
Create the most beautiful study materials using our templates.
Sign up to highlight and take notes. It’s 100% free.
