Question

(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 solution and why?
$S{c}^{3+},V^{3+},T{i}^{4+},M{n}^{2+}$
($Sc=21,V=23,Ti=22,Mn=25$)

Answer 1

a. $Cu$ is the only metal in the first transition series ($3d$ series) which exhibits $+1$ oxidation state more frequently. This is because the electronic configuration of $Cu$ is $3d^{10}4s^1$ and after losing one electron it acquires a stable $3d^{10}$ configuration.

b. Complexes of transition metal ions (e.g. $T{i}^{3+}and{V}^{3+}$) are usually coloured due to d-d transitons within the partly filled d orbitals. Although there is a partially filled d level in scandium ($\left[Ar\right]3d^{14}{s}^2$ ), when it forms $S{c}^{3+}$, it loses all three outer electrons so the 3d level is empty. So, although scandium is a member of the d block like titanium and vanadium, its ion $(S{c}^{3+}$) hasn't got any d electrons left to move around. $S{c}^{3+}$ complexes are therefore colourless because no visible light is absorbed.