Assign reason for the following :

${\mathrm{Sc}}^{3+}$ ion is colourless while ${\mathrm{Cr}}^{3+}$ is coloured.

Assign reasons for the following :

${\mathrm{Mn}}^{2+}$ compounds are more stable than ${\mathrm{Fe}}^{2+}$ compounds towards oxidation to $+3$ state.

Assign reasons for the following :

$\mathrm{Cu}\left(\mathrm{I}\right)$ compounds are white and diamagnetic while $\mathrm{Cu}\left(\mathrm{II}\right)$ compounds are coloured and paramagnetic.