The regular decrease (contraction) in the size of the atoms and ions with increasing atomic number is known as lanthanide contraction.
Lanthanoid Contraction: The steady decrease in the atomic and ionic radii of the transition metals as the atomic number increases. This is because of filling of 4f orbitals before the 5d orbitals. This contraction is size is quite regular. This is called lanthanoid contraction. It is because of lanthanoid contraction that the atomic radii of the second row of transition elements are almost similar to those of the third row of transition elements.
Thus, the regular decrease (contraction) in the size of the atoms and ions with increasing atomic number is known as lanthanide contraction.
Actinoid Contraction: The gradual decrease in the ionic radii with the increase in atomic no. is called actinoid contraction.
Explanation: It is due to the imperfect shielding of one 5f electrons by another in the same subshell. Along 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 results in contraction of the size.