CBSE Class 12-science Answered

Lanthanoid Contraction:

The regular contraction (decrease in the size) of the atoms and ions with increasing atomic number is known as lanthanide 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.

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.

Actinide contraction is greater from element to element than in lanthanide contraction. This is because,

The actinides are radioactive in nature, with comparatively less stable nuclei, because in actinides; the electrons are filled in 5f-shell, which is considered to have less shielding effect than the 4f-shell of lanthanide.

It is because, the 5f-shell are found to be more penetrating than the 4f-shell because the 4f-shell is found between 6s and 5p- orbitals which minimizes the tendency to overlap with ligands while in 5f-shell it is nearly comparable with the energies of 6d, 7s and 7p and hence the electrons of actinide ions are not shielded by the penultimate shell and are poorly effected by the crystal field so that the effective nuclear charge experienced by the electrons in valence shell of actinides is much higher than that experienced by the lanthanide. Hence the contraction becomes greater in actinides than in lanthanide.