Question

Discuss the consequences of lanthanide contraction.

Answer 1

Hint:
The lanthanide contraction is observed from La to Lu due to the increasing nuclear charge and due to the electrons entering the inner (n-2) f orbital.

Complete step by step answer:
- In simple words we can say that the decrease in size of lanthanide ions is due to an increase in atomic number which is known as lanthanide contraction.
- There are several consequences of lanthanide contraction found, let’s discuss about some of them:
- Electronegativity of atoms: Electronegativity increases across a period, in lanthanides also it is found to increase from La to Lu because the number of charges on the nucleus increases, which attract the bonding pair of electrons more strongly.
- Effect on the basic strength of hydroxides: As the size of lanthanides decreases from La to Lu, their basic strength also decreases because of the decrease in the ions from$C{{e}^{3+}}\text{ }to\text{ }L{{u}^{3+}}$,the covalent character of the hydroxides increases and hence, the basic strength decreases. Thus, we can say that $Lu{{\left( OH \right)}_{3}}$is the least basic and $La{{\left( OH \right)}_{3}}$is more basic .
- Atomic size of lanthanides: We can also see that the size of the atom that is present in the third transition series is nearly the same as that of the atom of the second transition series. For example, it is seen that the radius of Zr, Hf , Nb ,Ta are almost the same.
- Complex formation ability: lanthanides are having the smaller size but higher nuclear charge. This results in higher charge density and high nuclear charge, which will facilitate the acceptance of lone pairs of electrons from ligands. Therefore we can see that these has tendency to form coordinate complexes, this is found to increase from$L{{a}^{3+}}$to $L{{u}^{3+}}$.
- Ionization energy: It is observed that the Attraction of electrons by the nuclear charge is much higher and therefore the Ionization energy of 5d elements are much larger than 4d and 3d.

Note:
- It is important to note that the Lanthanide Contraction is caused by a poor shielding effect of the 4f electrons. We can define the Shielding effect as a phenomenon by which the inner shell electrons shield the outer shell electrons, so that they are not affected by the nuclear charge.
- Shielding effect of subshells is found in order of s>p>d>f. In lanthanides, f orbitals do shielding.