Oxidation States: The Rules (AQA AS Chemistry)

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

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

• 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:

Answers

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!