Question

Describe the general trends in the following properties of the first series (3d) of the transition elements: 

(i) Number of oxidation states exhibited. 

(ii) Formation of oxo metal ions. 

Answer 1

(i) In 3d-series all the elements show + 2 oxidation state except Sc (Sc⁺³). Oxidation states first in- creases from Sc to Mn due to increase in number of unpaired electrons and then decreases because pairing take place. Fe and Ni show zero oxidation state in metal carbonyls. 

(ii) All the metals except scandium form MO oxides which are ionic. The highest oxidation number in the oxides, coincide with the group number and is at- tained in Sc₂O₃ to Mn₂O₇. Beyond group 7, no higher oxides of iron above Fe₂O₃ are known. Besides the oxides, the oxocations stabilise  as VO₂, VIV as VO²⁺ and Ti as TiO²⁺. As the oxidation number of a metal increases, ionic character decreases. In case of Mn, Mn₂O, is a covalent green oil. Even CrO₃ and V₂O₅ have low melting points. In their higher oxides, the acidic character is predominant. Thus, Mn₂O₇ gives HMnO and CrO₃ gives H₂CrO₄ and H₂Cr₂O₇. V₂O₅ is however, amphoteric though mainly acidic and it gives VO₄^3- as well as to VO₂⁺ salt.