Consequences of Lanthanide Contraction

When electrons reach the inner f orbitals (n-2) and the nuclear charge increases, lanthanide contraction occurs. This charge causes the atomic size of tripositive lanthanide ions to decrease from La to Lu gradually. The constant decrease in size as the atomic number grows is known as lanthanide contraction. All 14 elements of the lanthanide series are affected by lanthanide contraction.

Lanthanide Contraction

Victor Goldschmidt, a Norwegian geochemist, coined the phrase lanthanide contraction. Lanthanide contraction is defined as a higher than predicted drop in the ionic radii of the elements in the lanthanide family. As a result, the components after lutetium grow significantly smaller.

The existence of f orbitals and f electrons with a low shielding effect causes lanthanide contraction. As a result, the outer 6s electrons have a relatively high nuclear charge and are more attracted to the nucleus. Hence, these elements’ atomic radii are reduced. This effect extends to additional elements, particularly post-lanthanide elements.

Consequences of Lanthanide Contraction

Atomic Size

The third transition series atom has roughly the same size as the second transition series atom. The radius of Zr, for example, is equal to the radius of Hf, and the radius of Nb is equal to the radius of Ta, and so on.

Difficulty in the Separation of Lanthanides

The chemical characteristics of Lanthanides are identical because their ionic radii differ relatively slightly. The separation of the element in its pure state becomes more complex due to this.

Effect on the Basic Strength of Hydroxides

The basic tensile strength decreases from Lanthanum to Lutetium as the chemical bond characteristic of the M-(OH) bond increases due to lanthanide contraction.

Complex Formation

The charge-to-radius ratio of lanthanides with 3+ oxidation states is higher. When compared to d-block elements, the capacity of lanthanides to form complexes is consequently hampered. Despite this, they form compounds with strong chelating agents, including EDTA, -diketones, oxime, and others. They can’t make P-complexes because they lack the ability to do so.

Electronegativity

From the components La through Lu, it gets more complicated.

Ionisation Energy

Because the nuclear charge draws electrons more vigorously than the 4d and 3d elements, the ionisation energy of 5d elements is much greater. Except for Pt and Au, all elements in the 5d series have a fully filled s-shell. 

From Hafnium through Rhenium, the ionisation energy is equivalent, then climbs with the number of shared d-electrons, with Gold and Iridium having the highest ionisation energy.

Effect of Lanthanide Contraction

The effect of lanthanide contraction is caused by the 4f electrons’ poor shielding of nuclear charge. Then the 6s electrons are therefore attracted closer to the nucleus, which results in a declined atomic radius.

In single-electron atoms, the average separation of an electron from the nucleus is defined by the subshell to which it belongs, and it decreases with increasing nucleus charge, resulting in a decrease in atomic radius. Meanwhile, in multi-electron atoms, higher electrostatic repulsion between electrons partially offsets the decrease in radius produced by a rise in nuclear charge.

As more electrons are delivered to the outside shells, a ‘shielding effect’ occurs, in which present electrons shield the outer electrons from nuclear charge by enabling them to experience less effective charge on the nucleus.

The atomic radius falls when a given subshell is filled in a period. The 4f subshell, which is filled across these elements, is not more efficient at sheltering the outer shell (n=5 and n=6) electrons; hence this effect is more prominent in the case of lanthanides. As a result, the shielding effect may be less effective in counteracting the reduction in radius induced by a growing nuclear charge.

Causes of Lanthanide Contraction

The average spacing of an electron from the nucleus in single-electron atoms is determined by the subshell to which it belongs, and it decreases as the nucleus’ charge increases, resulting in a decrease in atomic radius. However, in multi-electron atoms, the rise in nuclear charge causes a decrease in radius, which is somewhat offset by growing electrostatic repulsion between the electrons.

When extra electrons are added to the outer shells, the Shielding Effect protects the outer electrons from the nuclear charge by causing them to experience a reduced effective charge on the nucleus.

Shielding Effect on the Atomic Radii

The 4f electrons’ modest shielding effect produces the lanthanide. When inner shell electrons shelter electrons of the outer shell from being pulled to the nucleus’ charge, the shielding effect occurs.

As a result, if the shielding effect is poor, the positively charged nucleus attracts the outer shell electron to itself, and the radii of that atom shrink as the positive charge or atomic number increases. S-shell has a stronger shielding effect than the other subshells, whereas the f-shell has the least. The d and p shells are in the middle, with the p-shell having a stronger shielding effect than the d-shell.

Conclusion

Lanthanide contractions occur as a result of the failure of the f-orbit shields to balance the growing charges with the increased amount of atoms, resulting in the contractions. Because nuclear charge exists in the inner shell of electrons, the job of shielding here is straightforward – it protects the outer shell from the charges of the inner shell. The shields in f-block elements fail to protect the outer shell from the nuclear charge, implying that positively charged particles enter the outer shell. When positively charged particles and electrons interact, the atomic radius decreases as the number of atoms increases.