Question

Define lanthanoid contraction and mention its consequences?

Answer 1

Part 1: Lanthanides:

• Lanthanides are the inner transition elements of the modern periodic table i.e. the elements with atomic numbers from 58 to 71after the lanthanum(La).
• They are also called rare earth metals because they are very rare on earth.

Part 2: Lanthanide Contraction:

• The steady decrease in the atomic size of lanthanides on increasing the atomic number is due to the increasing the nuclear charge and electrons entering the inner $(\mathrm{n}-2)\mathrm{f}$orbital or anti penultimate shell. This steady decrease in the size with an increasing atomic number is called lanthanide contraction.

Part 3: Consequences of Lanthanide Contraction:

Atomic size:

• The size of the atom of the third transition series is closely the same as that of the atom of the second transition series. For example, the atomic radius of $\mathrm{Zr}=$ atomic radius of Hf and the atomic radius of $\mathrm{Nb}=$atomic radius of $\mathrm{Ta}$ etc.

Difficulty in the separation of lanthanides:

• As there is only a minute change in the ionic radii of lanthanides, their chemical properties are the same. This makes the separation very difficult.

Effect on the basic strength of hydroxides:

• The size of lanthanides decreases from $\mathrm{La}$ to $\mathrm{Lu}$, the covalent character of the hydroxides increases, and hence their basic strength decreases. For example, $\mathrm{Lu}{\left(\mathrm{OH}\right)}_3$ is said to be the least basic, and $\mathrm{La}{\left(\mathrm{OH}\right)}_3$ is more basic.

Complex formation:

• Due to the smaller size but higher nuclear charge, the tendency to form a coordinate compound.
• Complexity increases from the element ${\mathrm{La}}^{3+}\mathrm{to}{\mathrm{Lu}}^{3+}$