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Oxidation Number

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Chemistry

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.

Oxidation number rules

There are a few rules that can help and simplify the way we work out oxidation numbers.

  • The oxidation number of all uncombined elements is zero. The reason behind this is that the element has neither lost any electrons, nor gained any, and is therefore neutral. Examples of these uncombined elements are Zn, H, and Cl.
  • The oxidation number of monatomic ions is the same as the ionic charge. For example, Zn2+ has the oxidation number +2, and Cl- has the oxidation number -1.
  • The oxidation number of a neutral compound is zero. For example, in NaCl, the oxidation number of Na is +1 and the oxidation number of Cl is -1. Therefore +1+(-1) = 0, the sum of the oxidation numbers is 0.
  • The sum of the oxidation numbers in an ion is equal to the overall charge on the ion. Let’s take CO32- as an example. In this ion, C has an oxidation number of +4 and O = -2. However, there are three lots of oxygen atoms, so -2 x 3 = -6. Then we calculate the total sum: +4+(-6) = -2. Therefore, the sum of the atom's individual oxidation numbers is equal to the overall charge on the ion.
  • 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. Remember that electronegativity decreases down a group and increases across a period. For example, in F2O, F is given the oxidation number of (-1 x 2) = -2, and O has the oxidation number +2.

    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.

  • Group 1 elements all have the oxidation number +1.
  • Group 2 elements all have the oxidation number +2.
  • Aluminum always has the oxidation number +3.
  • Fluorine always has the oxidation number -1.
  • Hydrogen has the oxidation number +1, except in metal hydrides where it is -1.
  • Oxygen has the oxidation number -2, except in peroxides where it is -1, and in compounds with fluorine where it is +2.
  • Chlorine has the oxidation number -1. However, this changes in compounds with oxygen and fluorine.

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

Oxidation number exceptions

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.

Oxidation numbers and naming compounds

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.

Working out oxidation numbers with examples

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.

Oxidation Number - Key takeaways

  • Oxidation is the loss of electrons.
  • 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 atom's individual oxidation numbers in an ion is equal to the overall charge on the ion.
  • In an ion or a compound, the element that is more electronegative is given the negative oxidation number.
  • Many elements have the same oxidation number in all of their compounds.
  • In metal hydrides, hydrogen has an oxidation number of -1.
  • Oxygen has the oxidation number -1 in peroxides.
  • An element’s oxidation number is shown using roman numerals if it has more than one oxidation number.

Oxidation Number

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.

Final Oxidation Number Quiz

Question

What is oxidation?

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Answer

Oxidation is a process which involves the loss of electrons.

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Question

What is the oxidation number of uncombined elements such as Cl?

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Answer

Zero

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Question

What is the sum of the oxidation numbers of a neutral compound? 


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Answer

Zero

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Question

What is the oxidation number of a monatomic ion?

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Answer

The oxidation number of monatomic ions is the same as the ionic charge.

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Question

What is the sum of all the oxidation numbers in a polyatomic ion?

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Answer

The sum of the oxidation numbers of polyatomic ions is the same as the charge of the ion. 

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Question

In a compound, what is the oxidation number of group 1 elements?

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Answer

+1

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Question

In a compound, what is the oxidation number of group 2 elements?


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Answer

+2

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Question

What is the oxidation number of aluminum in all compounds?

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Answer

+3

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Question

What is the oxidation number of fluorine in all compounds?


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Answer

-1

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Question

What is the oxidation number of hydrogen in metal hydrides?

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Answer

-1

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Question

What is the oxidation number of oxygen in peroxides such as H2O2?


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Answer

-1

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Question


What is the oxidation number of oxygen in F2O?

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Answer

In F2O, the more electronegative element is fluorine, so it gains the negative oxidation number of -1. There are two atoms of fluorine, so the oxidation number is doubled to -2. Therefore, the oxidation number of oxygen is +2.

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Question


In an ion or compound, which element is given the negative oxidation number?

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Answer

The element that is more electronegative.

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Question

How are an element's oxidation numbers represented?

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Answer

An element’s oxidation number is shown using roman numerals if it has more than one oxidation number and this is indicated in the names of the compounds.

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Question

What is the oxidation number of sulfur in H2SO4?

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Answer

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.

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