Lanthanide contraction and its consequences

Lanthanide Contraction

Lanthanides are elements in which the extra electron, or the differentiating electron enters (n-2)f orbitals. These are also called as 'f' block elements because of the same reason. Since the (n-2) f orbitals lie comparatively deep within the kernels(being inner to the penultimate shell), these elements are also called inner-transition elements.

In lanthanides, the 4f orbitals get progressively filled up. The lanthanides start from Cerium (Z = 58) to Lutetium, Lu (Z = 71).

Lanthanide Contraction Definition

Radii of tripositive lanthanide cations. A look at the values of the radii of tripositive lanthanide cations (M³⁺) reveals that these values decrease as we move from La³⁺ to Lu³⁺ in the lanthanide series.

"The steady decrease in the values of the radii of M³⁺ cations in the lanthanide series is called as Lanthanide contraction."

In this series, the size of La is maximum and that of Lu is minimum. The decrease in size, though continuous, is not regular. The reason for lanthanide contraction can be traced to imperfect shielding of one 4f-electron by another in the same sub-shell.

Ionic Radii of Elements

The ionic radii of elements of the 4f group are as follows.

Ions(tripositive)	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
Radii (Å)	1.06	1.03	1.01	0.99	0.98	0.96	0.95	0.94	0.92	0.91	0.89	0.88	0.87	0.86	0.85

Note that the ionic radii of tripositive elements are decreasing as we proceed from La to Lu, thereby confirming the fact that there is a contraction in the size of these molecules as we proceed along the series.

Cause of Lanthanide Contraction

We know that as we proceed from one element to the next one in the lanthanide series, the nuclear charge (i.e, atomic number) increases by +1 for each element and the addition of the extra electron takes place in the 4f orbital. Again it is also known that the shielding of one electron in 4f orbital by the one residing in the same orbital is very poor.

Due to the negligible amount of mutual shielding effect between the electrons residing in 4f orbital, the increase in nuclear charge by +1 for each element in the lanthanide series brings the valence shell nearer to the nucleus and hence the size of M³⁺ cations goes on decreasing as we move from one element to the next one in the series.

Thus, lanthanide contraction plays a significant role in the chemistry of lanthanides. The important consequences of it are:

1. There is a steady decrease in the ionic size.
2. There is a slight increase in electronegativity of the trivalent ions.
3. The E^o values for M³⁺ + 3e → M_(g) increases regularly from lanthanum - 2.52 V to 2.25 V for lutetium.

Shielding Effect Trend

"When the attraction between the outermost electron and the nucleus decreases due to the presence of inner electrons, the effect is called the shielding effect."

On moving along the lanthanide series the number of 4f- electrons increases by one unit at each step and the imperfect shielding Z_eff increases causing a contraction in the electron cloud of the 4f-subshell. 

Ionic radii changed from 1.06 Å to 1.85 Å. 

Thus, lanthanide contraction plays a significant role in the chemistry of lanthanides. 

Consequences of Lanthanide Contraction

Lanthanide contraction plays an important role in determining the chemistry of lanthanides and heavier transition elements. Some important consequences of lanthanide contraction are:

1. Basic character of lanthanide hydroxides, M(OH)₃

There is a decrease in basic strength of the hydroxides of lanthanides with increase in atomic number. Thus, La(OH)₃ is the most basic while Lu(OH)₃ is the least basic. Due to lanthanide contraction, the decrease in size of M³⁺ cations increases the covalent character(i.e, decrease in ionic character) between the M³⁺ ion and OH^- ion(according to Fajan's rules), thereby reducing the basic character of the lanthanide hydroxides.

These differences in basicity are reflected in:

• Hydrolysis of ions
• Solubility of salts.
• Thermal decomposition and
• Formation of complexes by these elements.

2. Atomic radii of post-lanthanides elements

Normally in the same sub-group the atomic (covalent) radii increases as the atomic number increases. On the basis of this rule the atomic radii of the elements of 3rd transition series should be greater than those of the elements of 2nd transition series, but, although the atomic radius of La is greater than that of 'Y.

La = 1.69 Å and Y = 1.62 Å

Also, the atomic radii of Hf, Ta, W, etc. (elements of 3rd transition series) are not greater than those of Zr, Nb, Mo, Tc, etc. respectively(elements of 2nd transition series).

The expected increase in the atomic radii on proceeding from the elements of 2nd transition series to those of 3rd transition series is cancelled by the presence of 14 lanthanides (Ce with Z= 58 to Lu with Z = 71) between La (Z= 57) and Hf(Z = 72).

Consequently, the atomic radii of the elements of 3rd transition series become almost equal to those of the elements of the 2nd transition series. Due to the similarity in the size of the elements of the two series, the elements of 2nd and 3rd series resemble each other more closely that do the elements of the 1st and 2nd transition series.

3. High density of post-lanthanide elements

Because of lanthanide contraction, the atomic sizes of the post - lanthanide elements become very small. Consequently the packing of atoms in their metallic crystal becomes so compact that their densities are very high. Thus, while the densities of the elements of the second transition series are only slightly higher that those of the elements of the first series, the densities of the elements of the third transition series are almost double to those of the elements of the second transition series.

4. Occurrence of Y with heavy lanthanides

The crystal radii of Y³⁺ and Fr³⁺ are equal (Y³⁺ = 0.88Å  and Er³⁺ = 0.88Å). This similarity in ionic size of these two cations coupled with the equality in ionic charge (= +3 in both the ions) accounts for the invariable occurrence of Y with heavier lanthanides. The marked similarities in the crystal structure, chemical properties and solubility between Yttrium compounds and the corresponding compounds of the heavier lanthanides make it difficult to separate Yttrium from the heavier lanthanides. It is because of these similarities that Yttrium is regarded for all practical purposes as a member of the lanthanide series.