Transition elements usually exist in several different oxidation states and the oxidation states changes in units of one, e.g. Fe²⁺ and Fe⁺³, Cu⁺¹ and Cu⁺².

 

Scandium can have an oxidation number of (+II) if both s electrons are used for bonding and (+III) when two s and one d electrons are involved. Similarly all the elements show variable oxidation states depending upon the number of electrons available for bonding in their s and d sub-shells.

Illustration 5:Why do transition elements show variable oxidation states?
Solution:In the transition elements, the energies of (n-1)d orbitals and ns orbitals are very close. Hence electrons from both can participate in bonding.

Illustration 6:Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number.

OR

Compare the stability of +2 oxidation state of the elements of the first transition series.

Solution:The sum IE₁ + IE₂ increases. As a result the standard reduction potentials (E⁰) becomes less and less negative. Hence the tendency to form M²⁺ ion decreases. The greater stability of +2 state for Mn is due to half-filled d-subshell (d⁵), that for zinc is due to completely filled d-subshell (d¹⁰) and half that for nickel is due to highest negative enthalpy of hydration.
 

Exercise 3: