by Graeme Matthews

March 2, 2021

Being able to determine the oxidation numbers of atoms is an important skill to master in A-level Chemistry. This is because it will allow you to correctly write the formula of a compound and will allow you to correctly write an oxidation or reduction reaction. It will help you write a balanced redox equation in neutral, acidic or alkaline solution. And it will help you solve fuel cell and redox titrations problems.

In this article, I show you how to determine the oxidation state of an element in a compound or ion. 

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The Difference Between Fixed And Variable Oxidation States

The oxidation state of ion in A-level is the same as the charge on the ion in GCSE. The two terms can be used interchangeably. However, if you use the term oxidation state when referring to the charge on the ion then redox equations will make a lot more sense.

Group 1, 2 and 3 atoms all have fixed oxidation states. This means that the oxidation state doesn't change even though what it is bonded to in a compound or ion may change.

Sodium has an oxidation state of +1, The reason it has this oxidation states is that it equals the number of valance electrons in the outer shell of the atom. It can't have an oxidation state of +2 because it only has one valance electron in it's outer shell. Using this logic, magnesium has an oxidation state of +2 and aluminium has an oxidation state of +3.

The nonmetals have a variable oxidation state. A variable oxidation state means that the oxidation state of an atom depends on what it is bonded to in the compound or ion. 

At GCSE you probably learned that the halogens have a charge of -1. Chlorine has several oxidation states which vary from -1 to +7. The reason for this is that the chlorine atom has seven valence electrons in its outer shell. So, it can either gain one valence electron to complete its shell and have an oxidation state of -1. Or it can lose seven valence electrons to have an oxidation state of +7. 

The reason for this is because chlorine has a high electronegativity value which means that it can attract an electron to itself in a bond causing the oxidation state of -1. It also has a low ionisation energy and a large atomic radius which means that it can easily lose electrons to another atom in a bond giving it an oxidation state of up to +7.

The other elements that have a variable oxidation state are the d-block elements. At GCSE you probably learned that iron has two ions, one with a +2 charge and one with a +3 charge. Although the chemistry of d-block elements is complex, the reason for the two differently charged ions has to do with the number of valence electrons in the outer shell and how they fill the d-subshell.

Certain atoms have ions with a common oxidation state. A common oxidation state is an oxidation state that you are most likely to see the ion has in a ion or a compound.

The following image from Compound Interest shows the common oxidation states of the elements.

Note the following elements and their oxidation states as these are the ones that are most frequently seen at A-level.

  • Hydrogen has an oxidation number of -1 if bonded to a metal atom i.e. NaH or +1 if attached to a nonmetal atom i.e. H2O.
  • Flourine has an oxidation number of -1.
  • Oxygen has an oxidation number of -2 in an oxide i.e. H2O or MgO or -1 if attached to a peroxide i.e. H2O2 or Li2O2.

Summary

A fixed oxidation state is an oxidation state of an atom that doesn't change. Group 1,2 and 3 metals have fixed oxidation states that equal their group number on the periodic table. A variable oxidation state is an oxidation state of an atom that does change depending on what it is bonded to in an ion or a compound. Metalloids and non-metals have variable oxidation states.


The oxidation state of an atom depends on the number of valence electrons it has in it's outershell.

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Determining The Oxidation State Of An Atom In A Compound Or An Ion

To determine the oxidation state of an atom in a compound or an ion involves completing the following steps:

  1. Writing down the fixed oxidation states of Group 1, 2 and 3 elements and other common ions.
  2. Determining the overall charge on the compound or ion.
  3.  Writing the algebraic equation to determine the unknown oxidation state of the atom in the compound or ion.
  4. Determining the oxidation state of the unknown atom.
  5.  Checking to see if the value you obtain for the unknown oxidation state makes sense from a chemical viewpoint.

In the video below, I show you how to quickly determine the oxidation of an atom in a compound or an ion.

Summary

The oxidation state that you will be asked to determine in a compound or an ion will be that of an atom with a variable oxidation state. 


To determine the oxidation state write an algebraic equation that is the sum of the total number of each atom and it's oxidation state and set it equal to zero if it is a compound or the charge on the ion. Insert the values for the common oxidation state and solve for the unknown oxidation state.


Keep in mind that the oxidation states of metals are positive and can range from the most negative value being equal to the number of electrons you would have to add in order to get a full shell to the most positive value being when all of the valence electrons have been removed.

About the author 

Graeme Matthews

Graeme Matthews has a B.Sc and an M.Sc in Chemistry and a PhD in Adult Education. He has been teaching a combination of university level, college and A-level chemistry for over 23 years. He has taught over 10,000 chemistry students in his teaching career. He has a proven track record of helping students earn an A* on their A-level chemistry exams so they can attend the university of their choice.

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