Oxidation Number (Edexcel AS Chemistry)

• There are three definitions of oxidation and reduction  used in different branches of chemistry
• Oxidation and reduction can be used to describe any of the following processes

Definitions and Examples of Oxidation & Reduction

Use the acronym "Oil Rig" to help you remember the definitions of oxidation and reduction

Oxidation Number

• The oxidation number of an atom is the charge that would exist on an individual atom if the bonding were completely ionic
• It is like the electronic ‘status’ of an element
• Oxidation numbers are used to
  • Tell if oxidation or reduction has taken place
  • Work out what has been oxidised and/or reduced
  • Construct half equations and balance redox equations

Oxidation Numbers of Simple Ions

Answers:

a) 0     b) +3    c) +2   

d) -2    e) 0      f) +3

• So, in simple ions, the oxidation numbers of the atom is the charge on the ion:
  • Na⁺, K⁺, H⁺ all have an oxidation number of +1
  • Mg²⁺, Ca²⁺, Pb²⁺ all have an oxidation number of +2
  • Cl^–, Br^–, I^– all have an oxidation number of -1
  • O^2-, S^2- all have an oxidation number of -2

• Roman numerals are also used to indicate the oxidation state of ions
  • Iron(II) sulfate implies that the iron is in a +2 oxidation state, i.e. Fe²⁺ 
  • Iron(III) sulfate implies that the iron is in a +3 oxidation state, i.e. Fe³⁺ 
• More complicated examples include other atoms / ions as part of the formula 
  • Potassium manganate(VII) implies that the manganese is in a +7 oxidation
  • Potassium manganate(VII) contains the potassium ion K⁺ and the manganate ion MnO₄^– 
    • Since the oxygen in the manganate ion is in the -2 oxidation state, there is a total of -8 from the oxygen
    • The manganate ion has an overall -1 charge, which means that the manganese ion must be in the +7 oxidation state

Oxidation Number Rules

• A few simple rules help guide you through the process of determining the oxidation number of any element
• Remember, you are determining the oxidation number of a single atom
• The oxidation number (ox.no.) refers to a single atom in a compound

Oxidation Number 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

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

• From its position in the periodic table and / or
• The other element(s) present in the formula
• The oxidation numbers of all other atoms in their compounds can vary
• By following the oxidation number rules, the oxidation number 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 number

Oxidation Numbers & the Periodic Table