State the consequences of lanthanoid contraction. Explain the formation of coloured ions by lanthanoids.

(N/A) The consequences of lanthanoid contraction are as follows:
$1$. Similarity in atomic radii: The atomic radii of elements of the second and third transition series are similar (e.g.,$Zr-Hf$,$Nb-Ta$). This makes their separation difficult due to similar chemical properties.
$2$. Ionization enthalpies: The ionization enthalpies of the third transition series are generally higher than those of the second transition series.
$3$. Basic strength of hydroxides: The basic strength of lanthanoid hydroxides decreases from $La(OH)_{3}$ to $Lu(OH)_{3}$ due to the decrease in ionic size,which increases the covalent character.
$4$. Density: Elements of the third transition series exhibit abnormally high densities.
Regarding coloured ions:
Many trivalent lanthanoid ions $(M^{3+})$ are coloured in both solid and aqueous states. This is due to $f-f$ transitions. The absorption bands are narrow. Ions with $4f^{n}$ configuration have the same colour as those with $4f^{(14-n)}$ configuration (e.g.,$Nd^{3+}$ $(4f^{3})$ and $Er^{3+}$ $(4f^{11})$ are both red). Ions with $f^{0}$,$f^{7}$,and $f^{14}$ configurations (e.g.,$La^{3+}$,$Gd^{3+}$,$Lu^{3+}$) are colourless.