AQA A Level Chemistry复习笔记1.9.2 Oxidation States: The Rules

Oxidation States


Oxidation State Rules

  • A few simple rules help guide you through the process of determining the oxidation state of any element
  • Remember, you are determining the oxidation state of a single atom

Oxidation Rules Table


Molecules or Compounds

  • In molecules or compounds, the sum of the oxidation states on the atoms is zero

Oxidation State in Molecules or Compounds


  • Because CO2 is a neutral molecule, the sum of the oxidation states must be zero
  • For this, one element must have a positive oxidation state and the other must be negative

How do you determine which is the positive one?

  • The more electronegative species will have the negative value
  • Electronegativity increases across a period and decreases down a group
  • O is further to the right than C in the periodic table so it has the negative value

How do you determine the value of an element’s oxidation state?

  • From its position in the periodic table and/or
  • The other element(s) present in the formula
  • The oxidation states of all other atoms in their compounds can vary
  • By following the oxidation state rules, the oxidation state of any atom in a compound or ion can be deduced
  • The position of an element in the periodic table can act as a guide to the oxidation state

Oxidation States & the Periodic Table


  • Test your understanding on the following examples:

Worked Example

Deducing oxidation statesGive the oxidation state of the elements in bold in these compounds or ions:

a. P2O5

b. SO42-

c. H2S

d. Al2Cl6

e. NH3

f.  ClO2-


Are oxidation states always whole numbers?

  • The answer is yes and no
  • When you try and work out the oxidation state of sulfur in the tetrathionate ion S4O62- you get an interesting result!


The oxidation state of sulfur in S4O62- is a fraction

  • The fact that the oxidation state comes out to +2.5 does not mean it is possible to get half an oxidation state
  • This is only a mathematical consequence of four sulfur atoms sharing +10 oxidation state
  • Single atoms can only have an integer oxidation states, because you cannot have half an electron!

Exam Tip

Although there is a slight technical difference between the terms, oxidation state and oxidation number can be used interchangeably in your exams and examiners will know what you are talking about!