AQA A Level Chemistry

Revision Notes

6.2.5 Variable Oxidation States

Variable Oxidation States

  • Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
    • Variable oxidation states
    • Behave as catalysts
    • Form complex ions
    • Form coloured compounds

Ions of transition metals

  • Like other metals on the periodic table, the transition elements will lose electrons to form positively charged ions
  • However, unlike other metals, transition elements can form more than one positive ion
    • They are said to have variable oxidation states
  • Because of this, Roman numerals are used to indicate the oxidation state on the metal ion
    • For example, the metal sodium (Na) will only form Na+ ions (no Roman numerals are needed, as the ion formed by Na will always have an oxidation state of +1)
    • The transition metal iron (Fe) can form Fe2+ (Fe(II)) and Fe3+ (Fe(III)) ions
  • The table below shows the most common oxidation states of a few transition metals

Oxidation states of transition elements table

Chemistry of Transition Elements - Oxidation states of transition elements table, downloadable AS & A Level Chemistry revision notes

Coloured complex

  • Another characteristic property of transition elements is that their compounds are often coloured
    • For example, the colour of the [Cr(OH)6]3- complex (where oxidation state of Cr is +3) is dark green
    • Whereas the colour of the [Cr(NH3)6]3+ complex (oxidation state of Cr is still +3) is purple

Chemistry of Transition Elements - Coloured Transition Metal Complexes, downloadable AS & A Level Chemistry revision notes

Examples of some transition metal ions and their coloured complexes

Transition elements as catalysts

  • Since transition elements can have variable oxidation states, they make excellent catalysts
  • During catalysis, the transition element can change to various oxidation states by gaining electrons or donating electrons from reagents within the reaction
    • For example, iron (Fe) is commonly used as a catalyst in the Haber Process, switching between the +2 and +3 oxidation states
  • Substances can also be adsorbed onto their surface and activated in the process

Complex ions

  • Another property of transition elements caused by their ability to form variable oxidation states, is their ability to form complex ions
  • A complex ion is a molecule or ion, consisting of a central metal atom or ion, with a number of molecules or ions surrounding it
  • The molecules or ions surrounding the central metal atom or ion are called ligands
  • Due to the different oxidation states of the central metal ions, a different number and wide variety of ligands can form bonds with the transition element
    • For example, the chromium(III) ion can form [Cr(NH3)6]3+, [Cr(OH)6]3- and [Cr(H2O)6]3+ complex ions

Redox Potential & Oxidation State

Redox Potential & Oxidation State

Oxidation states of Vanadium

Oxidation states of Vanadium

Reduction of Tollens' Reagent

Reduction of Tollens’ Reagent


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