Variable Oxidation States

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Variable Oxidation States

All transition metals except Sc are capable of bivalency. The 4s electrons are first used and then 3d electrons.

There is a great variety of oxidation states but patterns can be found.

Reason: Close similarity in energy of 4s and 3d electrons. Calcium, for example, only has oxidation state number +2 in compounds due to ease at which electrons are lost from 4s, but any further loss would need much greater energy since the third electron is to be found in an inner shell.

Known oxidation states can be summarised by the table below.

Note: Mn can have an oxidation state of +7 due to the hypothetical loss of 7 electrons (4s2 3d5) - after this nuclear charge binds electrons more strongly. Mn has the maximum number of unpaired electrons available for bond formation. The number of unpaired electron decreases steadily on either side of Mn. Hence, the pattern shown below.

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All show oxidation state +2 (except Sc) due to loss of two 4s electrons.

All show +3, but rare in Ni and Cu.

Since, Transition metal ions are small they have a high charge density, therefore, display similar properties to Aluminium.

  1. Stability of higher oxidation states decreases from left to right. A possible reason is the increase in nuclear charge.
  2. Relative stability of +2 and +3 state

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a) The increasing stability of +2 across the period is caused by the greater difficulty of removing a third electron as nuclear charge increases.

b) Mn2+/Mn3+ and Fe2+/Fe3+ have stabilities that do not fit in this pattern. This can be explained by the stability of 3d5 found in Fe3+ and Mn2+.

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