# Given reasons for the following:(i) The transition metals generally form coloured compounds.(ii) $E^{\circ}$ value for $\left ( Mn^{3+}/Mn^{2+} \right )$ is highly positive than that for $\left ( Cr^{3+}/Cr^{2+} \right )$ couple.(iii) The chemistry of actinoids elements is not so smooth as that of the lanthanoids.

(i) Colour is due to the presence of incomplete d- subshell. During these transtions, they emit raditions corresponding to some wavelength in the visible region spectrum.

(ii) It is so because the third ionisation energy of Mn is very high $\left (d^{5}to d^{4} \right )$

(iii) This is due to the much smaller energy difference between 5f,6d and 7s subshells of actinoids than 4f, 5d and 6s subshells of lanthanoids.

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