Cause of Lanthanide Contraction

The atomic and ionic size of an element’s atoms  decreases with increase in atomic number from lanthanum(57) to lutetium (71) is known as lanthanide 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 the atomic numbers from 57-71.

Lanthanide contractions happen due to the failure of shielding effect  of the f orbit to balance the increasing charges with the increasing atomic number, resulting in the contractions.

Understanding the Lanthanides:

Lanthanides are a part of f-block elements, which entails the entrance of differentiating electrons in the antepenultimate shell. Electronic configuration of f-block elements is (n−2) f(0−14)(n−1)d(0−1)ns². The above-mentioned differentiating elements can enter the orbits of 4f or 5f, which gives them the identifications of lanthanides and actinides. Lanthanides and actinides are also referred to as rare earth elements.

The list of elements in the Lanthanides is as follows

Lanthanide Contractions

Lanthanide contractions happen due to the inability of the shields of the f block, which fails to balance the increasing charges with the increase in the number of atoms, resulting in the contractions. There is a presence of nuclear charge in the inner shell of the electrons – the role of the shielding here is simple, that is, to protect the outer shell from the charges of the inner shell. In f-block elements, the shields fail to protect the outer shell from the nuclear charge, which implies that the positively charged particles enter the outer shell. When the interaction among the positively charged particles and electrons happens, it reduces atomic radii with the increase in the number of atoms. Below mentioned are the causes and effects of Lanthanide contractions:

Lanthanide contraction causes and consequences

The interaction of positive nuclear charge to the outer shell of the orbit, leading to compression of the electrons.

Increase in the number of atoms in the elements

Inability to shield 4f electrons by the 11 electrons 5s2, 5p6, 5d1, and 6s2.

Weak shielding among different elements of the 4f block.

All the elements of the 4f block out, together with their interaction with positive nuclear charge, create a cumulative effect, which results in a larger contraction in the elements of the 4f block.

The sudden contraction between 57-58 elements of the periodic table.

Effects of Lanthanide Contraction

Lanthanide Separation

Due to their similar sizes, separating the lanthanides is difficult, but their properties differ slightly due to lanthanide contraction (such as the ability to form complexes). They are distinguished by differences in their properties.

Hydroxide Strength

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

Transition

The atomic radii of second-row transition elements are nearly identical to those of third-row transition elements because the increase in size as you move down the group from second to third transition elements is canceled out by the decrease in size due to lanthanide contraction.

Conclusion

Lanthanide elements have specific uses, and understanding lanthanide contractions makes it simpler to understand the application of these elements with the rationale of using the same.  Though certain lanthanides have limited applications, members of this category may be found in everything from normal lighters to TV tubes of CRT and from colorful glasses to atomic reactor control rods.