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Answer
Lanthanoid Contraction : In the lanthanoid series, with increasing atomic number, the atomic and ionic radii decrease from one element to another. This regular decrease (or contraction) is known as lanthanoid contraction.
Cause of Lanthanoid Contraction : In the lantha- noid series, as we move from one element to another, the nuclear charge increase by unit with atomic number. The new electron is added to the same inner 4f-sub-shell. As a result, the attraction as the electrons by the nucleus increases and this tends to decrease the size. Further as the new electron is added into the f-sub-shell, the shielding of one 4f-electron by another is imperfect, because of the very diffused shape of the f-orbitals. This imperfect shielding is unable to counterbalance the effect of the increased nuclear charge. Hence with increasing atomic number and nuclear charge, the effective nuclear charge experienced by each 4f-electron increases. As a result, the whole of 4f electron shell contracts at each successive element, though the decrease is very small. This results in gradual decrease in size of lanthanoids with increasing atomic number. The sum of the successive reduction gives the total lanthanoid contraction.
In Lanthanoid, the decrease of radius for fourteen elements (Ce, Z = 58 to Lu, Z = 71) is 15 pm (from 188 to 173).
Consequences of Lanthanoid Contraction
(i) Resemblance in size of elements belonging to some group of second and third transition series :
Normally in the same group, atomic radius increases as the atomic number increases (e.g., Sc -> Y -> La ) However, the size of the atoms belonging to 2nd and 3rd transition series after lanthanum (Z = 57) are similar to each other due to the effect of lanthanoid contraction.
It is thus a direct consequence of lanthanoid contraction that the elements of the second and third transition series resemble each other much more closely than do the elements of the first and second transition series.
(ii) Difficulty in separation of lanthanoids : Because of very small change in radii of lanthanoids, their chemical properties are quite similar. Thus, it is very difficult to separate the elements in pure state. Recently, methods based on repeated fractional crystallization or ion exchange techniques, which take the advantage of slight differences in their properties (like solubility, complex ion formation, hydration, etc.) arising from very slight size differences of their trivalent ions have been used.
(iii) Basicity differences : As the size of lanthanoid ions decreases from$La ^{3+}$ to $Lu ^{3+}$, the covalent character of hydroxides increases and hence the basic strength of hydroxides decreases with increase in atomic number. Thus, La (OH) 3 is most basic while Lu (OH) 3 is the least basic.
(iv) Reduction Potential and Metallic Character : The standard electrode (reduction) potentials of the lanthanoid ions become less negative across the series. Thus, their reducing power decreases in going from Ce to Lu. The highly negative $E ^0$ values indicate these elements be highly electropositive metals capable of displacing hydrogen from water.
$2 M +6 H _2 O \longrightarrow 2 M ( OH )_3+3 H _{2(g)}$
The $M ( OH )_3$ are ionic and basic in character. These hydroxides are stronger than $Al ( OH )_3$ and weaker than $Ca ( OH )_2$. The basic strength decreases in going from La to Lu.
(v) Solubility of Compounds: The fluorides, oxides, hydroxides, carbonates, phosphatets, chromates and oxalates of lanthanoids are largely insoluble in water. On the other hand, the halides other than fluorides; nitrates, acetates, perchlorates and salts of oxoacids of lanthanoids follow the pattern of solubility of salts of group 2 elements. However, lanthanoid sulphates unlike the sulphates of group 2 elements are soluble in water.
(vi) Complex Formation : The lanthanoid ions $\left( M ^{3+}\right)$ have a high charge but their size being large, they do not form complexes very radily.
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