(a) Which metal in the first transition series (3d series) exhibits + 1 oxidation state most frequently and why?

(b) Which of the following cations are coloured in aqueous solutions and why?

Sc³⁺, V³⁺, Ti⁴⁺,Mn²⁺

(At. nos. Sc= 21, V = 23, Ti = 22, Mn= 25)

(a) Cu is the only metal in the first transition series (3d series) that exhibits +1 oxidation state more frequently.

The electronic configuration of Cu is 3d¹⁰ 4s¹. So, when it loses one ‘s’ electron, it acquires a stable 3d¹⁰ configuration. Thus, it shows +1 oxidation state more frequently.

(b) We know that the colour of cations depend upon the number of unpaired electrons present in the d-orbital.

So, the electronic configuration of the following cations are:-

1. Sc (Atomic number 21) = [Ar]3d¹4s² and Sc³⁺ = 3d⁰4s⁰. As discussed, since d-orbital is empty, it is colourless.

2. V (atomic number 23) = [Ar]3d³4s² and V³⁺ = 3d²4s⁰. As d-orbital is having 2 unpaired electrons, it undergoes d-d transition and gives green colour.

3. Ti (Atomic number 22) = [Ar]3d²4s² and Ti⁴⁺ = 3d⁰4s⁰. As ‘d’ orbital is empty, it is colourless.

4. Mn (Atomic number 25) = [Ar]3d⁵4s² and Mn²⁺ = 3d⁵4s⁰. As ‘d’ orbital has 5 unpaired electrons, it undergoes d-d transition and gives pink colour.

Thus, V3+ and Mn2+ ions are coloured in their aqueous solution due to presence of unpaired ‘d’ electrons.