Lanthanide Contraction and its Consequence

Lanthanide Contraction and Consequence

Lanthanide Contraction

The regular decrease in the ionic radii of trivalent lanthanide ions along the lanthanide series from Ce³⁺ to Lu³⁺ is known as lanthanide contraction. We can explain lanthanide contraction in terms of effective nuclear charge (Z^*). On adding electron to the well shielded 4f orbitals along the lanthanide series, the Z^*_ion (= Z − S) increases by 0.15 and we know that,
Hence, the regular decrease in size of trivalent lanthanide ions occurs along the series. the screening constant value (S) for an atom is greater than that of a corresponding ion, therefore, the decrease in atomic radius with increase in atomic number is less marked in comparison to the decrease in ionic radius.

Consequence of Lanthanide Contraction

1. Normally in a group, atomic radius increases as the atomic number increases e.g. Sc(1.64Å), Y(1.80Å) and La(1.88 Å) but due to lanthanide contraction, the atomic radius of elements just following i.e. Hf and onwards is close to the elements just above them in their respective groups- Zr/Hf, Nb/Ta, Mo/W etc. have identical sizes. Hence, 4d & 5d series resemble each other more closely than do the 3d & 4d series.

2. The densities of 3d series are only slightly higher than those of 4d series while the densities of 5d series are nearly two times the densities of corresponding 4d seires.

3. Melting points of Hf, Ra & Ac are higher than they would have been otherwise.

4. In the subgroup, ionization potential decreases but in W(74) and succeeding elements right upto Pb(82), ionization potential are higher than in the corresponding elements above them.

5. Due to the smaller size and higher nuclear charge, the tendency to complex formation increases from the La³⁺ to Lu³⁺.

6. The similar ionic charge, size and outer electronic configuration account for the occurrence of Yttrium with heavy lanthanides Yb³⁺ (0.88Å), Ho³⁺ (0.89Å), Er³⁺ (0.88Å). Due to lanthanide contraction, the size of lanthanide ions regularly decreases and at Ho & Er, the size of Yb³⁺ is reached. Hence Yb is obtained with heavy lanthanides.

7. Because of regular size decrease, the properties like basicity change regularly and so there is an appreciable difference in properties of the 1st and 2nd member. Larger the size, the greater is the ease with which electron can be lost i.e. basicity decreases from Ce³⁺ to Lu³⁺. Therefore, Lu(OH)₃ is said to be least basic, and La(OH)₃ is said to be more basic. More basic ions hydrolyse less readily and more basic salts decompose less readily.