25.2    Oxidation  States  of  Transition Elements

Variable oxidation states

• 1. One characteristic property of transition elements is that they have variable oxidation states.
  2. For example, iron can exist in 0, +2 or +3 oxidation state.

• This is because the 4s and 3d electrons have very similar energy levels, therefore the transition element can easily lose or gain electrons to form ions/compounds of roughly the same stability

• All the possible oxidation states of the transition elements are shown below, the most common ones are bolded

• An oxidation state of +2 implies that two electrons have been lost from the neutral The same goes for other oxidation states.

• The maximum oxidation state of a compound can be obtained by counting the number of 4s electrons and the number of unpaired 3d electrons. Only copper breaks this pattern

• 1. For small oxidation states, the transition element normally exists as simple Mª⁺ ions.
  2. For large oxidation states, the transition element normally exists in oxo- This is because the ions formed would have a very high charge density and it will polarise adjacent molecules(water, oxygen) to form the oxo- compounds.
  3. For example, Cr⁶⁺ does not exist, chromium with oxidation state +6 exists as CrO4²⁻.

Redox reactions of transition elements.

Note:

1) There is no point learning this here, you should already have a decent knowledge on the Fe³⁺/Fe²⁺, MnO4⁻/Mn²⁺ and Cr2O7²⁻/Cr³⁺ systems up to this point of  the syllabus.

2) You need to know how to:

1. construct redox equations
2. calculate oxidation states
3. calculate the amount of substance(mass/volume) from a titration that involves oxidation  and reduction
4. select suitable oxidising and reducing agents
5. calculate cell potentials