Question

On the basis of Lanthanoid contraction, explain the following:

A) Nature of bonding in $L{a}_2{O}_{3}andL{u}_2{O}_3$ .

B) Trends in the stability of oxo salts of lanthanoids from $LatoLu$.

C) Stability of the complexes of lanthanoids.

D) Radii of $4d$ and $5d$ block elements.

E) Trends in acidic character of lanthanoid oxides.

Answer 1

A)

• According to Fajan’s rule, as the size of cation decreases and size of anion increases, the covalent character increases.
• And size of lanthanoid ions decreases from$LatoLu$ due to lanthanoid contraction.
• Since, size of cations decrease thus, as per Fajans’ rule, covalent character in Lanthanoid oxides increase from $LatoLu$
• And therefore, $L{a}_2{O}_3$is comparatively ionic in nature while, $L{u}_2{O}_3$ is comparatively covalent in nature.

B)

• Oxo salts of lanthanoids contains lanthanoid element as cation and oxygen as anion.
• Due to lanthanoid contraction, the size of lanthanoids decreases from $LatoLu$, and according to Fajan’s rule, smaller is the size of cation greater will be its polarisation power.
• Thus, as size decreases from La to Lu, polarisation power of cation increases and it will distort the electron cloud more effectively on anion such that the bond strength will decrease and stability will also decrease.
• Therefore, as size of lanthanoids decreases, stability of oxo salts of lanthanoids from $LatoLu$ also decreases.

C)

• The size of lanthanoids decreases from $LatoLu$ due to lanthanoid contraction, and a small cation with greater charge attracts the ligands better and thus complexes become stable.
• Therefore, as the size of cation decreases from $LatoLu$, the stability of complexes of lanthanoids will increase.

(D)

• The shielding effect of f orbitals is very poor due to which in case of $5d$ block elements, the nuclear charge increases on valence shell electrons and size of these elements decreases to a great extent.
• This phenomenon is called lanthanoid contraction.
• Therefore, radii of $4dand5d$ block elements are almost similar.

E)

• As size of lanthanoid ions decreases and covalent character increases from $LatoLu$, thus, acidic character of lanthanoid oxides increases.