Lanthanoid Contraction and Its Consequences

Lanthanides are the modern periodic table’s rare earth elements, with atomic numbers ranging from 58 to 71 after lanthanum. Since the occurrence of these elements is extremely uncommon, they are known as rare earth metals.

The lanthanide series is the set of elements in which the 4f sublevel is full. All of these elements are metals (specifically, transition metals). They share several characteristics.

However, there is significant disagreement over where the lanthanides begin and terminate. Lanthanum and lutetium  both are d-block elements and not f-block elements. Nonetheless, the two components share features with the other elements in the group.

When addressing generic lanthanide chemistry, the lanthanides are denoted by the chemical symbol Ln. 

Lanthanides are a part of lanthanide series, rare earth metals are a part of rare earth elements

 Inner transition metals and lanthanoids are all names for this group of elements.

Properties of lanthanides

• Physical qualities are similar across the series.
• Adoption of the +3-oxidation state is the most common. They can also have an oxidation state of +2 or +4, while some lanthanides are most stable in the +3 oxidation state.
• Adoption of coordination numbers larger than six that is around eight to nine in compounds.
• Across the series, there is a tendency for the coordination number to decrease.
• A tendency of binding towards elements that are more electronegative (such as O or F) 
• There is less reliance on ligands.
• Rapid ligand exchange occurs in ionic complexes.

Lanthanide Contraction

As nuclear charge increases and electrons reach the inner (n-2) f orbitals, the atomic size or ionic radius of tri positive lanthanide ions decreases progressively from La to Lu. Lanthanide contraction refers to the consistent reduction in size with increasing atomic number.

The atomic radius trend seen in the Lanthanide series is described by this contraction. It is a crucial phenomenon in defining the properties of lanthanide series elements.

The Lanthanide Contraction holds true for all the  14 elements in the Lanthanide series, they are-

Cerium(Ce), Praseodymium(Pr), Neodymium(Nd), Promethium(Pm), Samarium(Sm), Europium(Eu), Gadolinium(Gd), Terbium(Tb), Dysprosium(Dy), Holmium(Ho), Erbium(Er), Thulium(Tm), Ytterbium(Yb), and Lutetium are all members of this series (Lu).

According to the Lanthanide Contraction theory,  the atomic radius of these elements decreases with a gradual rise in their atomic number .

Reasons for Lanthanide Contractions

• Lanthanoid 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. 
• Since 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. 

Causes of Lanthanide Contraction

• The interaction of positive nuclear charge with the outer shell of the orbit, resulting in electron compression.
• The number of atoms in the elements grows.
• The 11 electrons’ inability to shelter 4f electrons from 5s2, 5p6, 5d1, and 6s2 subshells and shells.
• Weak shielding effect of the  various components of  4f block elements.
• The interaction of all the elements of the 4f block with a positive nuclear charge has a cumulative impact, thus resulting in a bigger contraction in the 4f block elements
• There is an abrupt contraction of the periodic table elements 57-58, due to the above mentioned phenomenon. 

Consequences of Lanthanide Contractions

The major consequences of Lanthanide Contractions are as follows: –

• Atomic size –

An atom in the third transition series is roughly the same size as an atom in the second transition series. For example, the radius of Zr equals the radius of Hf, and the radius of Nb equals the radius of Ta, and so on.

• Difficulty in the separation of lanthanides –

 Lanthanides have chemical properties that are comparable because their ionic radii differ just a little. In the pure form, this makes element separation difficult.

• The influence of lanthanide size on the basic strength of hydroxide –

 As the size of the lanthanide decreases from La to Lu, the covalent nature of the hydroxides increases,  hence their basic strength decreases.

•  As a result, La(OH)3 has higher basicity, whereas Lu(OH)3 has the lowest basicity.
• The reason to generate coordinates is a result of the smaller size and enhanced nuclear charge. From La3+ to Lu3,
•  the level of complexity increases.
• Electronegativity rises from La to Lu.

• Ionization energy – 

The ionization energy of 5d elements is substantially higher than that of 4d and 3d elements because the nuclear charge attracts electrons much more strongly. All elements in the 5d series, except Pt and Au, have a filled s-shell. Elements with the same ionization energy range from Hafnium to Rhenium, and following ionization energy increases with the number of shared d-electrons, with iridium and gold having the greatest ionization energy.

• Complex formation – 

Lanthanides in the 3+ oxidation state have a higher charge-to-radius ratio. When compared to d-block elements, lanthanides’ capacity to form complexes is consequently limited.

Conclusion

The continuous drop in an element’s atoms and ions – resulting from an increasing number of atoms – is known as lanthanoid contraction. The element series in which such contractions occur is known as the lanthanide series. These elements in this series consist of 15 elements on the periodic table, with atomic numbers from 57-71.