What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

Explanation

The steady decrease in the size of lanthanoid ions `(M^(3+))` with the increase in atomic number is called lanthanoid contraction :
(a) Separation of lanthanoids is difficult due to lanthanoid contraction because due to this contraction, their ionic radii are very small.
(b) The basic strength of hydroxides decreases from `"La(OH)"_(3)` to `"Lu(OH)"_(3)`. Due to lanthanoid contraction, size of `M^(3+)` ions decreases and there is increase in the covalent character in M-OH bond.