Edexcel A Level Chemistry:复习笔记2.1.1 Oxidation Number

• 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
  • O2-, S2- all have an oxidation number of -2

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