The given statement is true. From element to element, the actinoid contraction is greater than the lanthanoid contraction due to poor shielding by 5f-electrons in actinoids than that by 4f-electrons in the lanthanoids.
The steady decrease in the atomic and ionic radii of the transition metals as the atomic number increases. This is because of the filling of 4f orbitals before the 5d orbitals. This contraction in size is quite regular. This is called lanthanoid contraction. The gradual decrease in the ionic radii with the increase in atomic no. is called actinoid contraction.
- It is due to the imperfect shielding of one 5f electron by another in the same subshell. Along with the series, the nuclear charge and no. of 5f electrons increases by one unit; however, due to imperfect shielding of 5f orbitals, the effective nuclear charge increases, which result in contraction of the size.