On the basis of Lanthanoid contraction,explain the following :
$(i)$ Nature of bonding in $La_2O_3$ and $Lu_2O_3$.
$(ii)$ Trends in the stability of oxo salts of lanthanoids from $La$ to $Lu$.
$(iii)$ Stability of the complexes of lanthanoids.
$(iv)$ Radii of $4d$ and $5d$-block elements.
$(v)$ Trends in acidic character of lanthanoid oxides.

(N/A) $(i)$ From $La$ to $Lu$,the ionic size decreases. Hence,$La_2O_3$ is more ionic,while $Lu_2O_3$ has more covalent character.
$(ii)$ Due to the decrease in ionic size from $La$ to $Lu$,the stability of oxo-salts decreases.
$(iii)$ As the ionic size decreases,the charge density increases,which leads to an increase in the stability of lanthanoid complexes.
$(iv)$ Due to lanthanoid contraction,the atomic radii of $4d$ and $5d$ transition series elements are nearly identical.
$(v)$ As the ionic size decreases from $La$ to $Lu$,the covalent character of the oxides increases,leading to an increase in their acidic character.