Oxidation States: The Rules (AQA A Level Chemistry)

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:

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 S₄O₆^2- you get an interesting result!

The oxidation state of sulfur in S₄O₆^2- 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!