d and f Block Elements
8.0 Lanthanoid Contraction and its consequence
8.0 Lanthanoid Contraction and its consequence
- Constant steady decrease in the atomic and ionic radii of lanthanoids from lanthanum to lutetium with increase in atomic number is called lanthanoid contraction.
- There is no sharp decrease in the atomic radii but ionic radii decreases sharply from $La-Lu$.
Causes of Lanthanoid Contraction:
- As the atomic number increases addition of electrons can take place one by one in $4f$ of inner shell.
- The extent of shielding created by electrons in $4f$ is weaker than electrons in $d$ subshell.
- The shielding of electrons in $f$-subshell is weak due to its diffused shape.
- Due to the addition of electrons relatively at inner position effective nuclear charge increases.
- As the effective nuclear charge increases and screening effect decreases atomic and ionic radii gets contracted more than expected.
- $Th$ combine decrease in the atomic radii $Ce$ to $Lu$ is of $10\ pm$ and in ionic radii is of $18\ pm$.
- Decrease in the atomic or ionic radii from element to element is less than expected but collective decrease is appreciable.
