IB DP Chemistry: SL复习笔记9.1.2 Deducing Oxidation Numbers

Deducing Oxidation Numbers

  • 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 state

Oxidation Numbers & The Periodic Table


  • Test your understanding on the following examples:

Worked Example

State the oxidation number of the atoms in blue in these compounds or ions

a) P2O5

b) SO42-

c) H2S

d) Al2Cl6

e) NH3

f) ClO2-



Are oxidation numbers always whole numbers?

  • The answer is yes and no
  • When you try and work out the oxidation of sulfur in the tetrathionate ion S4O62- you get an interesting result!


The oxidation number of sulfur in S4O62- is a fraction

  • The fact that the oxidation number comes out to +2.5 does not mean it is possible to get half an oxidation number
  • This is only a mathematical consequence of four sulfur atoms sharing +10 oxidation number
  • Single atoms can only have an integer oxidation number, because you cannot have half an electron!