9.1.1 Oxidation & reduction

• 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 Table

Oxidation Numbers

• 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

Atoms and simple ions

• The oxidation number is the number of electrons which must be added or removed to become neutral
• The oxidation number is always written with the charge before the number

Oxidation Number of Simple Ions Table

Answers:

a) 0     b) +3    c) +2    d) -2    e) 0    f) +3

• So, in simple ions, the oxidation number 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

Molecules or Compounds

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

Oxidation Number in Molecules or Compounds Table

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

• from its position in the periodic table and/or
• the other element(s) present in the formula

• Many atoms, such as S, N and Cl, can exist in a variety of oxidation states
• The oxidation number of these atoms can be calculated by assuming that the oxidation number of the other atom is fixed
• Here are six rules to deduce the oxidation number of an element

Oxidation Number Rules Table