8.1 Effects of lanthanoid contraction

  d and f Block Elements

• Lanthanides form common $+3$ oxidation states.

• Ionic radii of tripositive lanthanoids ions decreases from $L{a^{ + 3}}$ to $L{u^{ + 3}}$.

• Lanthanides form trihydroxide i.e., $Ln{(OH)_3}$.

• As the ionic radii decreases metallic character of these metals also decreases.

• Ionic character in $M-OH$ bond decreases and covalent character increases in the hydroxides of lanthanides from $Ln{(OH)_3}$-$Lu{(OH)_3}$.

• As $Ln{(OH)_3}$ is more ionic undergo maximum dissociation and becomes more basic whereas $Lu{(OH)_3}$ is least stable due to more covalent character.

• According to Fajan's rule as the size of cation in metal hydroxides decreases their dissociation decreases.

• The decreasing order of basicity is $La{(OH)_3}$ > $Ce{(OH)_3}$ >....... > $Lu{(OH)_3}$.

• The elements arranged in third transition series after lanthanides are called post lanthanides.

• In any group of periodic table as we move from top to bottom the atomic radii of elements increases due to addition of electrons in new shell.

• The atomic radii of elements in third transition series after lanthanides are similar to the atomic radii of elements in same group of previous period.

• The pairs of elements $Zn-Hf$ (group $4$), $Nb-Ta$ (group $5$), $Mo-W$ (group $6$), $Tc-Re$ (group $7$) etc. have almost identical atomic radii.

• These pairs of elements have identical valence electrons and properties hence they are called chemical twins.

• The elements of second and third transition series arranged after lanthanides have close properties due to lanthanoid contraction.

• Misch Metal: (Alloy of lanthanum, iron and traces of $S$, $C$, $Ca$ and $Al$) is used to manufacture bullets and shells.